Add option to be notified when a followed user posts (#13546)

* Add bell button

Fix #4890

* Remove duplicate type from post-deployment migration

* Fix legacy class type mappings

* Improve query performance with better index

* Fix validation

* Remove redundant index from notifications

app/serializers/rest/notification_serializer.rb

@@ -11,6 +11,6 @@ class REST::NotificationSerializer < ActiveModel::Serializer
   end
 
   def status_type?
-    [:favourite, :reblog, :mention, :poll].include?(object.type)
+    [:favourite, :reblog, :status, :mention, :poll].include?(object.type)
   end
 end