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”

db/migrate/20190927232842_add_voters_count_to_polls.rb

@@ -0,0 +1,5 @@
+class AddVotersCountToPolls < ActiveRecord::Migration[5.2]
+  def change
+    add_column :polls, :voters_count, :bigint
+  end
+end