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/Providers/EventServiceProvider.php

@@ -21,9 +21,15 @@
 
 namespace App\Providers;
 
+use App\Application;
 use App\Listeners\LogAuthenticationFailure;
 use App\Listeners\LogAuthenticationSuccess;
 use App\Listeners\OnUserRegistration;
+use App\Observers\ApplicationObserver;
+use App\Observers\UserObserver;
+use App\Observers\VacancyObserver;
+use App\User;
+use App\Vacancy;
 use Illuminate\Auth\Events\Failed;
 use Illuminate\Auth\Events\Login;
 use Illuminate\Auth\Events\Registered;
@@ -74,7 +80,5 @@ class EventServiceProvider extends ServiceProvider
     public function boot()
     {
         parent::boot();
-
-        //
     }
 }