Add voters count support (#11917)

* Add voters count to polls

* Add ActivityPub serialization and parsing of voters count

* Add support for voters count in WebUI

* Move incrementation of voters count out of redis lock

* Reword “voters” to “people”

app/serializers/rest/poll_serializer.rb

@@ -2,7 +2,7 @@
 
 class REST::PollSerializer < ActiveModel::Serializer
   attributes :id, :expires_at, :expired,
-             :multiple, :votes_count
+             :multiple, :votes_count, :voters_count
 
   has_many :loaded_options, key: :options
   has_many :emojis, serializer: REST::CustomEmojiSerializer