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/serializers/post_wordpress_serializer.rb

@@ -2,11 +2,9 @@
 class PostWordpressSerializer < BasicPostSerializer
   attributes :post_number
 
-  include UrlHelper
-
   def avatar_template
     if object.user
-      absolute object.user.avatar_template
+      UrlHelper.absolute object.user.avatar_template
     else
       nil
     end

app/serializers/user_wordpress_serializer.rb

@@ -1,12 +1,10 @@
 class UserWordpressSerializer < BasicUserSerializer
 
-  include UrlHelper
-
   def avatar_template
     if Hash === object
-      absolute User.avatar_template(user[:username], user[:uploaded_avatar_id])
+      UrlHelper.absolute User.avatar_template(user[:username], user[:uploaded_avatar_id])
     else
-      absolute object.avatar_template
+      UrlHelper.absolute object.avatar_template
     end
   end