FIX: do not preload topic list for new topic/message routes (#18959)

This commit fixes the issue where the sub-category topic list was not
loading for new-topic routes. Since we do not need to preload topic
lists for new topic/message routes this commit adds a no-op controller
that prevents topic lists pre loading and at the same time fixes the sub
category topics not loading issue.

app/controllers/new_topic_controller.rb

@@ -0,0 +1,5 @@
+# frozen_string_literal: true
+
+class NewTopicController < ApplicationController
+  def index; end
+end