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>

app/controllers/users_controller.rb

@@ -354,10 +354,7 @@ class UsersController < ApplicationController
     user = User.new(new_user_params) if user.nil?
 
     # Handle API approval
-    if user.approved
-      user.approved_by_id ||= current_user.id
-      user.approved_at ||= Time.zone.now
-    end
+    ReviewableUser.set_approved_fields!(user, current_user) if user.approved?
 
     # Handle custom fields
     user_fields = UserField.all