DEPRECATION WARNING: Directly inheriting from ActiveRecord::Migration
is deprecated. Please specify the Rails release the migration was
written for:
class MigrationClass < ActiveRecord::Migration[4.2]
(called from require at bin/rails:4)
We can use the `config.authorize_with` option, so we don't need to copy
the controller in order to load and authorize resource.
Besides, only administrators can upload images, so we don't need to
track the image's user id.
