Make notifications cancellable

This commit makes certain notifications cancellable. This enables notifications to be sent conditionally based on the user's choice.

app/Notifications/VacancyClosed.php

@@ -9,10 +9,12 @@ use Illuminate\Notifications\Notification;
 use Illuminate\Queue\SerializesModels;
 
 use App\Vacancy;
+use App\Facades\Options;
+use App\Traits\Cancellable;
 
 class VacancyClosed extends Notification implements ShouldQueue
 {
-    use Queueable, SerializesModels;
+    use Queueable, SerializesModels, Cancellable;
 
     protected $vacancy;
 
@@ -26,15 +28,9 @@ class VacancyClosed extends Notification implements ShouldQueue
       $this->vacancy = $vacancy;
     }
 
-    /**
-     * Get the notification's delivery channels.
-     *
-     * @param  mixed  $notifiable
-     * @return array
-     */
-    public function via($notifiable)
+    public function optOut($notifiable)
     {
-        return ['mail'];
+        return Options::getOption('notify_vacancystatus_change') !== 1;
     }
 
     /**