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/providers/github/session.go

@@ -0,0 +1,56 @@
+package github
+
+import (
+	"encoding/json"
+	"errors"
+	"strings"
+
+	"github.com/markbates/goth"
+)
+
+// Session stores data during the auth process with Github.
+type Session struct {
+	AuthURL     string
+	AccessToken string
+}
+
+// GetAuthURL will return the URL set by calling the `BeginAuth` function on the Github provider.
+func (s Session) GetAuthURL() (string, error) {
+	if s.AuthURL == "" {
+		return "", errors.New(goth.NoAuthUrlErrorMessage)
+	}
+	return s.AuthURL, nil
+}
+
+// Authorize the session with Github and return the access token to be stored for future use.
+func (s *Session) Authorize(provider goth.Provider, params goth.Params) (string, error) {
+	p := provider.(*Provider)
+	token, err := p.config.Exchange(goth.ContextForClient(p.Client()), params.Get("code"))
+	if err != nil {
+		return "", err
+	}
+
+	if !token.Valid() {
+		return "", errors.New("Invalid token received from provider")
+	}
+
+	s.AccessToken = token.AccessToken
+	return token.AccessToken, err
+}
+
+// Marshal the session into a string
+func (s Session) Marshal() string {
+	b, _ := json.Marshal(s)
+	return string(b)
+}
+
+func (s Session) String() string {
+	return s.Marshal()
+}
+
+// UnmarshalSession will unmarshal a JSON string into a session.
+func (p *Provider) UnmarshalSession(data string) (goth.Session, error) {
+	sess := &Session{}
+	err := json.NewDecoder(strings.NewReader(data)).Decode(sess)
+	return sess, err
+}