Oauth2 consumer (#679)

* initial stuff for oauth2 login, fails on:
* login button on the signIn page to start the OAuth2 flow and a callback for each provider
Only GitHub is implemented for now
* show login button only when the OAuth2 consumer is configured (and activated)
* create macaron group for oauth2 urls
* prevent net/http in modules (other then oauth2)
* use a new data sessions oauth2 folder for storing the oauth2 session data
* add missing 2FA when this is enabled on the user
* add password option for OAuth2 user , for use with git over http and login to the GUI
* add tip for registering a GitHub OAuth application
* at startup of Gitea register all configured providers and also on adding/deleting of new providers
* custom handling of errors in oauth2 request init + show better tip
* add ExternalLoginUser model and migration script to add it to database
* link a external account to an existing account (still need to handle wrong login and signup) and remove if user is removed
* remove the linked external account from the user his settings
* if user is unknown we allow him to register a new account or link it to some existing account
* sign up with button on signin page (als change OAuth2Provider structure so we can store basic stuff about providers)

* from gorilla/sessions docs:
"Important Note: If you aren't using gorilla/mux, you need to wrap your handlers with context.ClearHandler as or else you will leak memory!"
(we're using gorilla/sessions for storing oauth2 sessions)

* use updated goth lib that now supports getting the OAuth2 user if the AccessToken is still valid instead of re-authenticating (prevent flooding the OAuth2 provider)

vendor/github.com/markbates/goth/gothic/gothic.go

@@ -0,0 +1,219 @@
+/*
+Package gothic wraps common behaviour when using Goth. This makes it quick, and easy, to get up
+and running with Goth. Of course, if you want complete control over how things flow, in regards
+to the authentication process, feel free and use Goth directly.
+
+See https://github.com/markbates/goth/examples/main.go to see this in action.
+*/
+package gothic
+
+import (
+	"errors"
+	"fmt"
+	"net/http"
+	"os"
+
+	"github.com/gorilla/mux"
+	"github.com/gorilla/sessions"
+	"github.com/markbates/goth"
+)
+
+// SessionName is the key used to access the session store.
+const SessionName = "_gothic_session"
+
+// Store can/should be set by applications using gothic. The default is a cookie store.
+var Store sessions.Store
+var defaultStore sessions.Store
+
+var keySet = false
+
+func init() {
+	key := []byte(os.Getenv("SESSION_SECRET"))
+	keySet = len(key) != 0
+	Store = sessions.NewCookieStore([]byte(key))
+	defaultStore = Store
+}
+
+/*
+BeginAuthHandler is a convienence handler for starting the authentication process.
+It expects to be able to get the name of the provider from the query parameters
+as either "provider" or ":provider".
+
+BeginAuthHandler will redirect the user to the appropriate authentication end-point
+for the requested provider.
+
+See https://github.com/markbates/goth/examples/main.go to see this in action.
+*/
+func BeginAuthHandler(res http.ResponseWriter, req *http.Request) {
+	url, err := GetAuthURL(res, req)
+	if err != nil {
+		res.WriteHeader(http.StatusBadRequest)
+		fmt.Fprintln(res, err)
+		return
+	}
+
+	http.Redirect(res, req, url, http.StatusTemporaryRedirect)
+}
+
+// SetState sets the state string associated with the given request.
+// If no state string is associated with the request, one will be generated.
+// This state is sent to the provider and can be retrieved during the
+// callback.
+var SetState = func(req *http.Request) string {
+	state := req.URL.Query().Get("state")
+	if len(state) > 0 {
+		return state
+	}
+
+	return "state"
+
+}
+
+// GetState gets the state returned by the provider during the callback.
+// This is used to prevent CSRF attacks, see
+// http://tools.ietf.org/html/rfc6749#section-10.12
+var GetState = func(req *http.Request) string {
+	return req.URL.Query().Get("state")
+}
+
+/*
+GetAuthURL starts the authentication process with the requested provided.
+It will return a URL that should be used to send users to.
+
+It expects to be able to get the name of the provider from the query parameters
+as either "provider" or ":provider".
+
+I would recommend using the BeginAuthHandler instead of doing all of these steps
+yourself, but that's entirely up to you.
+*/
+func GetAuthURL(res http.ResponseWriter, req *http.Request) (string, error) {
+
+	if !keySet && defaultStore == Store {
+		fmt.Println("goth/gothic: no SESSION_SECRET environment variable is set. The default cookie store is not available and any calls will fail. Ignore this warning if you are using a different store.")
+	}
+
+	providerName, err := GetProviderName(req)
+	if err != nil {
+		return "", err
+	}
+
+	provider, err := goth.GetProvider(providerName)
+	if err != nil {
+		return "", err
+	}
+	sess, err := provider.BeginAuth(SetState(req))
+	if err != nil {
+		return "", err
+	}
+
+	url, err := sess.GetAuthURL()
+	if err != nil {
+		return "", err
+	}
+
+	err = storeInSession(providerName, sess.Marshal(), req, res)
+
+	if err != nil {
+		return "", err
+	}
+
+	return url, err
+}
+
+/*
+CompleteUserAuth does what it says on the tin. It completes the authentication
+process and fetches all of the basic information about the user from the provider.
+
+It expects to be able to get the name of the provider from the query parameters
+as either "provider" or ":provider".
+
+See https://github.com/markbates/goth/examples/main.go to see this in action.
+*/
+var CompleteUserAuth = func(res http.ResponseWriter, req *http.Request) (goth.User, error) {
+
+	if !keySet && defaultStore == Store {
+		fmt.Println("goth/gothic: no SESSION_SECRET environment variable is set. The default cookie store is not available and any calls will fail. Ignore this warning if you are using a different store.")
+	}
+
+	providerName, err := GetProviderName(req)
+	if err != nil {
+		return goth.User{}, err
+	}
+
+	provider, err := goth.GetProvider(providerName)
+	if err != nil {
+		return goth.User{}, err
+	}
+
+	value, err := getFromSession(providerName, req)
+	if err != nil {
+		return goth.User{}, err
+	}
+
+	sess, err := provider.UnmarshalSession(value)
+	if err != nil {
+		return goth.User{}, err
+	}
+
+	user, err := provider.FetchUser(sess)
+	if err == nil {
+		// user can be found with existing session data
+		return user, err
+	}
+
+	// get new token and retry fetch
+	_, err = sess.Authorize(provider, req.URL.Query())
+	if err != nil {
+		return goth.User{}, err
+	}
+
+	err = storeInSession(providerName, sess.Marshal(), req, res)
+
+	if err != nil {
+		return goth.User{}, err
+	}
+
+	return provider.FetchUser(sess)
+}
+
+// GetProviderName is a function used to get the name of a provider
+// for a given request. By default, this provider is fetched from
+// the URL query string. If you provide it in a different way,
+// assign your own function to this variable that returns the provider
+// name for your request.
+var GetProviderName = getProviderName
+
+func getProviderName(req *http.Request) (string, error) {
+	provider := req.URL.Query().Get("provider")
+	if provider == "" {
+		if p, ok := mux.Vars(req)["provider"]; ok {
+			return p, nil
+		}
+	}
+	if provider == "" {
+		provider = req.URL.Query().Get(":provider")
+	}
+	if provider == "" {
+		return provider, errors.New("you must select a provider")
+	}
+	return provider, nil
+}
+
+func storeInSession(key string, value string, req *http.Request, res http.ResponseWriter) error {
+	session, _ := Store.Get(req, key + SessionName)
+
+	session.Values[key] = value
+
+	return session.Save(req, res)
+}
+
+func getFromSession(key string, req *http.Request) (string, error) {
+	session, _ := Store.Get(req, key + SessionName)
+
+	value := session.Values[key]
+	if value == nil {
+		return "", errors.New("could not find a matching session for this request")
+	}
+
+	return value.(string), nil
+}