Store remote votes URI (#10158)

* Store remote votes URI

* Add spec for accepting remote votes

* Make poll vote id generation work the same way as follows

db/migrate/20190304152020_add_uri_to_poll_votes.rb

@@ -0,0 +1,5 @@
+class AddUriToPollVotes < ActiveRecord::Migration[5.2]
+  def change
+    add_column :poll_votes, :uri, :string
+  end
+end