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/workers/poll_expiration_notify_worker.rb

@@ -11,12 +11,12 @@ class PollExpirationNotifyWorker
     # Notify poll owner and remote voters
     if poll.local?
       ActivityPub::DistributePollUpdateWorker.perform_async(poll.status.id)
-      NotifyService.new.call(poll.account, poll)
+      NotifyService.new.call(poll.account, :poll, poll)
     end
 
     # Notify local voters
     poll.votes.includes(:account).map(&:account).select(&:local?).each do |account|
-      NotifyService.new.call(account, poll)
+      NotifyService.new.call(account, :poll, poll)
     end
   rescue ActiveRecord::RecordNotFound
     true