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/models/poll.rb

@@ -16,6 +16,7 @@
 #  created_at      :datetime         not null
 #  updated_at      :datetime         not null
 #  lock_version    :integer          default(0), not null
+#  voters_count    :bigint(8)
 #
 
 class Poll < ApplicationRecord