declare controller classes as abstract

Closes #1056

Merge request studip/studip!626

app/controllers/course/forum/forum_controller.php

 <?php
 
-class ForumController extends StudipController {
+abstract class ForumController extends StudipController {
     protected $with_session = true;
 
     /* * * * * * * * * * * * * * * * * * * * * * * * * */

app/controllers/module/mvv_controller.php

@@ -5,7 +5,7 @@
  * @since       3.5
  */
 
-class MVVController extends AuthenticatedController
+abstract class MVVController extends AuthenticatedController
 {
     /**
      * The maximum number of items listed on a page.

app/controllers/settings/settings.php

@@ -15,7 +15,7 @@
 
 require_once 'lib/messaging.inc.php';
 
-class Settings_SettingsController extends AuthenticatedController
+abstract class Settings_SettingsController extends AuthenticatedController
 {
     // Stores message which shall be send to the user via email
     protected $private_messages = [];