FEATURE: New 'Reviewable' model to make reviewable items generic

Includes support for flags, reviewable users and queued posts, with REST API
backwards compatibility.

Co-Authored-By: romanrizzi <romanalejandro@gmail.com>
Co-Authored-By: jjaffeux <j.jaffeux@gmail.com>

plugins/poll/plugin.rb

@@ -419,7 +419,7 @@ after_initialize do
   end
 
   on(:approved_post) do |queued_post, created_post|
-    if queued_post.post_options["is_poll"]
+    if queued_post.payload["is_poll"]
       created_post.validate_polls(true)
     end
   end