fixes for failed cron maintenance tasks

* update rails scripts for 5.2
* system dmails should be scoped to system
* fix broken url generator in forum notices

config/application.rb

@@ -1,9 +1,6 @@
 require_relative 'boot'
 require 'rails/all'
 
-if defined?(Bundler)
-  Bundler.require(:default, Rails.env)
-end
 Bundler.require(*Rails.groups)
 
 require_relative "danbooru_default_config"