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/user.go

@@ -196,6 +196,11 @@ func (u *User) IsLocal() bool {
 	return u.LoginType <= LoginPlain
 }
 
+// IsOAuth2 returns true if user login type is LoginOAuth2.
+func (u *User) IsOAuth2() bool {
+	return u.LoginType == LoginOAuth2
+}
+
 // HasForkedRepo checks if user has already forked a repository with given ID.
 func (u *User) HasForkedRepo(repoID int64) bool {
 	_, has := HasForkedRepo(u.ID, repoID)
@@ -397,6 +402,11 @@ func (u *User) ValidatePassword(passwd string) bool {
 	return subtle.ConstantTimeCompare([]byte(u.Passwd), []byte(newUser.Passwd)) == 1
 }
 
+// IsPasswordSet checks if the password is set or left empty
+func (u *User) IsPasswordSet() bool {
+	return !u.ValidatePassword("")
+}
+
 // UploadAvatar saves custom avatar for user.
 // FIXME: split uploads to different subdirs in case we have massive users.
 func (u *User) UploadAvatar(data []byte) error {
@@ -947,6 +957,12 @@ func deleteUser(e *xorm.Session, u *User) error {
 		return fmt.Errorf("clear assignee: %v", err)
 	}
 
+	// ***** START: ExternalLoginUser *****
+	if err = RemoveAllAccountLinks(u); err != nil {
+		return fmt.Errorf("ExternalLoginUser: %v", err)
+	}
+	// ***** END: ExternalLoginUser *****
+
 	if _, err = e.Id(u.ID).Delete(new(User)); err != nil {
 		return fmt.Errorf("Delete: %v", err)
 	}
@@ -1190,6 +1206,11 @@ func GetUserByEmail(email string) (*User, error) {
 	return nil, ErrUserNotExist{0, email, 0}
 }
 
+// GetUser checks if a user already exists
+func GetUser(user *User) (bool, error) {
+	return x.Get(user)
+}
+
 // SearchUserOptions contains the options for searching
 type SearchUserOptions struct {
 	Keyword  string