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>

db/migrate/20190315170411_add_index_to_reviewable_histories.rb

@@ -0,0 +1,5 @@
+class AddIndexToReviewableHistories < ActiveRecord::Migration[5.2]
+  def change
+    add_index :reviewable_histories, :created_by_id
+  end
+end