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.

app/controllers/application_controller.rb

@@ -27,16 +27,8 @@ class ApplicationController < ActionController::Base
   respond_to :html
   helper_method :current_budget
 
-  before_action :set_user_for_ckeditor_pictures
-
   private
 
-    def set_user_for_ckeditor_pictures
-      if request.path == '/ckeditor/pictures' && request.request_method == 'POST'
-        params['user_id'] = current_user.id
-      end
-    end
-
     def authenticate_http_basic
       authenticate_or_request_with_http_basic do |username, password|
         username == Rails.application.secrets.http_basic_username && password == Rails.application.secrets.http_basic_password