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/post_migrate/20190121203023_drop_queued_post_id_from_user_actions.rb

@@ -0,0 +1,9 @@
+class DropQueuedPostIdFromUserActions < ActiveRecord::Migration[5.2]
+  def up
+    remove_column :user_actions, :queued_post_id
+  end
+
+  def down
+    add_column :user_actions, :queued_post_id, :integer
+  end
+end

db/post_migrate/20190123171817_drop_queued_posts.rb

@@ -0,0 +1,5 @@
+class DropQueuedPosts < ActiveRecord::Migration[5.2]
+  def up
+    drop_table :queued_posts
+  end
+end