Avoid more models in old migrations

Although these migrations run fine today, they could break in the future
if changes are made to the current model code.

db/migrate/051_add_status_to_user.rb

@@ -1,6 +1,9 @@
 require "migrate"
 
 class AddStatusToUser < ActiveRecord::Migration[4.2]
+  class User < ActiveRecord::Base
+  end
+
   def self.up
     create_enumeration :user_status_enum, %w[pending active confirmed suspended deleted]