When someone isn't a superuser any more, they shouldn't see the reporsts any more either.
Here we delete the report notifications from a user when that user gets updated from being a superuser to a non-superuser.
* These are the first small steps for issue 2034 "Reports should send a notification to admins".
* I added a new type of notification "pleroma:report" to the the database manually (a migration will need to be written later)
* I added the new type to the notification_controller
* I made the view return the notification. It doesn't include the report itself (yet)
