fix: add constraint actions to db structure

This commit adds several missing "cascade delete" actions to relationships on database tables. This effectively fixes errors while trying to delete user accounts because of pending child records.

Additionally, the observers for applications and vacancies were removed, since they are now obsolete.

The account deletion system was also refactored.

app/Http/Controllers/UserController.php

@@ -33,6 +33,7 @@ use App\Http\Requests\UpdateUserRequest;
 use App\Notifications\ChangedPassword;
 use App\Notifications\EmailChanged;
 use App\Traits\DisablesFeatures;
+use App\Traits\HandlesAccountDeletion;
 use App\Traits\ReceivesAccountTokens;
 use App\User;
 use Google2FA;
@@ -44,7 +45,7 @@ use Spatie\Permission\Models\Role;
 
 class UserController extends Controller
 {
-    use ReceivesAccountTokens;
+    use HandlesAccountDeletion;
 
     public function showUsers()
     {