DEV: Upgrading Discourse to Zeitwerk (#8098)

Zeitwerk simplifies working with dependencies in dev and makes it easier reloading class chains. 

We no longer need to use Rails "require_dependency" anywhere and instead can just use standard 
Ruby patterns to require files.

This is a far reaching change and we expect some followups here.

app/models/topic_embed.rb

@@ -1,8 +1,5 @@
 # frozen_string_literal: true
 
-require_dependency 'nokogiri'
-require_dependency 'url_helper'
-
 class TopicEmbed < ActiveRecord::Base
   include Trashable