oauth config: Use GITLAB_URL env variable

In order to use oauth from gitlab CE instance one has to be
able to provide the URL of the instance.

closes #209 (https://gitlab.com/commento/commento/issues/209)

api/oauth_gitlab.go

@@ -35,6 +35,8 @@ func gitlabOauthConfigure() error {
 		},
 		Endpoint: gitlab.Endpoint,
 	}
+	gitlabConfig.Endpoint.AuthURL = os.Getenv("GITLAB_URL") + "/oauth/authorize"
+	gitlabConfig.Endpoint.TokenURL = os.Getenv("GITLAB_URL") + "/oauth/token"
 
 	gitlabConfigured = true