/**
  * Logs in a user.
  * @return mixed response
  */
 public function actionLogin()
 {
     if (!Yii::$app->user->isGuest) {
         return $this->goHome();
     }
     $model = new LoginForm();
     if ($model->load(Yii::$app->request->post()) && $model->login()) {
         return $this->goBack();
     } else {
         return $this->render('login', ['model' => $model]);
     }
 }