Add base implementation of extendable events classes

Modified server deletion to use internal event handlers from the
Eloquent models themselves. Also added a few preliminary event handlers
in the `Pterodactyl\Events\<USer|Server> namespace that users can hook
into in EventServiceProvider to perform their own actions as they
please (such as push notifications and such).

app/Providers/EventServiceProvider.php

@@ -12,11 +12,7 @@ class EventServiceProvider extends ServiceProvider
      *
      * @var array
      */
-    protected $listen = [
-        'Pterodactyl\Events\ServerDeleted' => [
-            'Pterodactyl\Listeners\DeleteServerListener',
-        ],
-    ];
+    protected $listen = [];
 
     /**
      * Register any other events for your application.