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)

models/error.go

@@ -847,3 +847,43 @@ func IsErrUploadNotExist(err error) bool {
 func (err ErrUploadNotExist) Error() string {
 	return fmt.Sprintf("attachment does not exist [id: %d, uuid: %s]", err.ID, err.UUID)
 }
+
+//  ___________         __                             .__    .____                 .__          ____ ___
+//  \_   _____/__  ____/  |_  ___________  ____ _____  |  |   |    |    ____   ____ |__| ____   |    |   \______ ___________
+//   |    __)_\  \/  /\   __\/ __ \_  __ \/    \\__  \ |  |   |    |   /  _ \ / ___\|  |/    \  |    |   /  ___// __ \_  __ \
+//   |        \>    <  |  | \  ___/|  | \/   |  \/ __ \|  |__ |    |__(  <_> ) /_/  >  |   |  \ |    |  /\___ \\  ___/|  | \/
+//  /_______  /__/\_ \ |__|  \___  >__|  |___|  (____  /____/ |_______ \____/\___  /|__|___|  / |______//____  >\___  >__|
+//          \/      \/           \/           \/     \/               \/    /_____/         \/               \/     \/
+
+// ErrExternalLoginUserAlreadyExist represents a "ExternalLoginUserAlreadyExist" kind of error.
+type ErrExternalLoginUserAlreadyExist struct {
+	ExternalID    string
+	UserID        int64
+	LoginSourceID int64
+}
+
+// IsErrExternalLoginUserAlreadyExist checks if an error is a ExternalLoginUserAlreadyExist.
+func IsErrExternalLoginUserAlreadyExist(err error) bool {
+	_, ok := err.(ErrExternalLoginUserAlreadyExist)
+	return ok
+}
+
+func (err ErrExternalLoginUserAlreadyExist) Error() string {
+	return fmt.Sprintf("external login user already exists [externalID: %s, userID: %d, loginSourceID: %d]", err.ExternalID, err.UserID, err.LoginSourceID)
+}
+
+// ErrExternalLoginUserNotExist represents a "ExternalLoginUserNotExist" kind of error.
+type ErrExternalLoginUserNotExist struct {
+	UserID        int64
+	LoginSourceID int64
+}
+
+// IsErrExternalLoginUserNotExist checks if an error is a ExternalLoginUserNotExist.
+func IsErrExternalLoginUserNotExist(err error) bool {
+	_, ok := err.(ErrExternalLoginUserNotExist)
+	return ok
+}
+
+func (err ErrExternalLoginUserNotExist) Error() string {
+	return fmt.Sprintf("external login user link does not exists [userID: %d, loginSourceID: %d]", err.UserID, err.LoginSourceID)
+}