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/controllers/topics_controller.rb

@@ -1,14 +1,5 @@
 # frozen_string_literal: true
 
-require_dependency 'topic_view'
-require_dependency 'promotion'
-require_dependency 'url_helper'
-require_dependency 'topics_bulk_action'
-require_dependency 'discourse_event'
-require_dependency 'rate_limiter'
-require_dependency 'topic_publisher'
-require_dependency 'post_action_destroyer'
-
 class TopicsController < ApplicationController
   requires_login only: [
     :timings,