public function actionAddQuestion()
 {
     $courseId = Yii::$app->request->post('courseId');
     $content = Yii::$app->request->post('content');
     $userId = Yii::$app->user->id;
     $model = new Question();
     if ($model->addData($userId, $content, $courseId)) {
         echo Json::encode('true');
         return;
     } else {
         echo Json::encode('false');
         return;
     }
 }