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>

spec/models/notification_spec.rb

@@ -269,7 +269,7 @@ describe Notification do
           topic: post.topic
         ))
 
-        PostAction.act(user, post, PostActionType.types[:like])
+        PostActionCreator.like(user, post)
       end.to change { Notification.count }.by(2)
 
       expect(Notification.filter_by_display_username_and_type(