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

lib/url_helper.rb

@@ -1,6 +1,6 @@
-module UrlHelper
+class UrlHelper
 
-  def is_local(url)
+  def self.is_local(url)
     url.present? && (
       Discourse.store.has_been_uploaded?(url) ||
       !!(url =~ /^\/assets\//) ||
@@ -9,15 +9,15 @@ module UrlHelper
     )
   end
 
-  def absolute(url, cdn = Discourse.asset_host)
+  def self.absolute(url, cdn = Discourse.asset_host)
     url =~ /^\/[^\/]/ ? (cdn || Discourse.base_url_no_prefix) + url : url
   end
 
-  def absolute_without_cdn(url)
-    absolute(url, nil)
+  def self.absolute_without_cdn(url)
+    self.absolute(url, nil)
   end
 
-  def schemaless(url)
+  def self.schemaless(url)
     url.sub(/^https?:/, "")
   end