Don't send notifications to ghost users.

We already skip sending notifications to blocked users. Simply add ghost users
to this list.

app/services/notification_service.rb

@@ -483,6 +483,7 @@ class NotificationService
 
     users = users.to_a.compact.uniq
     users = users.reject(&:blocked?)
+    users = users.reject(&:ghost?)
 
     users.reject do |user|
       global_notification_setting = user.global_notification_setting