FEATURE: move migrate_to_new_scheme into a background job

- new hidden site setting 'migrate_to_new_scheme' (defaults to false)
- new rake tasks to toggle migration to new scheme
- FIX: migrate_to_new_scheme also works with CDN
- PERF: improve perf of the DbHelper.remap method
- REFACTOR: UrlHelper is now a class

app/controllers/topics_controller.rb

@@ -5,8 +5,6 @@ require_dependency 'topics_bulk_action'
 require_dependency 'discourse_event'
 
 class TopicsController < ApplicationController
-  include UrlHelper
-
   before_filter :ensure_logged_in, only: [:timings,
                                           :destroy_timings,
                                           :update,
@@ -81,7 +79,7 @@ class TopicsController < ApplicationController
 
     perform_show_response
 
-    canonical_url absolute_without_cdn("#{Discourse.base_uri}#{@topic_view.canonical_path}")
+    canonical_url UrlHelper.absolute_without_cdn("#{Discourse.base_uri}#{@topic_view.canonical_path}")
   rescue Discourse::InvalidAccess => ex
 
     if current_user