Allow sites to bootstrap the error page.

This will display working dropdowns and such even if the page is a 404.

app/controllers/application_controller.rb

@@ -596,6 +596,11 @@ class ApplicationController < ActionController::Base
     end
 
     def build_not_found_page(status = 404, layout = false)
+      if SiteSetting.bootstrap_error_pages?
+        preload_json
+        layout = 'application' if layout == 'no_ember'
+      end
+
       category_topic_ids = Category.pluck(:topic_id).compact
       @container_class = "wrap not-found-container"
       @top_viewed = TopicQuery.new(nil, except_topic_ids: category_topic_ids).list_top_for("monthly").topics.first(10)

config/site_settings.yml

@@ -1103,6 +1103,9 @@ developer:
     choices:
       - 'default'
       - 'lograge'
+  bootstrap_error_pages:
+    hidden: true
+    default: false
 
 embedding:
   feed_polling_enabled: