Simplify CKEditor authorization

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.

db/migrate/20180813141443_create_ckeditor_assets.rb

@@ -10,8 +10,6 @@ class CreateCkeditorAssets < ActiveRecord::Migration
       t.integer :width
       t.integer :height
 
-      t.integer :user_id
-
       t.timestamps null: false
     end
 

db/schema.rb

@@ -302,7 +302,6 @@ ActiveRecord::Schema.define(version: 20180813141443) do
     t.string   "type",              limit: 30
     t.integer  "width"
     t.integer  "height"
-    t.integer  "user_id"
     t.datetime "created_at",                   null: false
     t.datetime "updated_at",                   null: false
   end