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

script/discourse

@@ -152,10 +152,6 @@ WHERE table_schema='public' and (data_type like 'char%' or data_type like 'text%
     require File.expand_path(File.dirname(__FILE__) + "/../config/environment")
   end
 
-  def schemaless(url)
-    url.gsub(/^https?:/, "")
-  end
-
 end
 
 DiscourseCLI.start(ARGV)