FEATURE: allow long polling to go to a different url

Added the site setting long_polling_base_url , this allows you
to farm long polling to a different server.

This setting is very important if a CDN is serving dynamic content.

config/initializers/04-message_bus.rb

@@ -2,6 +2,14 @@ MessageBus.site_id_lookup do
   RailsMultisite::ConnectionManagement.current_db
 end
 
+MessageBus.extra_response_headers_lookup do |env|
+  {
+    "Access-Control-Allow-Origin" => Discourse.base_url,
+    "Access-Control-Allow-Methods" => "GET, POST",
+    "Access-Control-Allow-Headers" => "X-SILENCE-LOGGER, X-Shared-Session-Key"
+  }
+end
+
 MessageBus.user_id_lookup do |env|
   user = CurrentUser.lookup_from_env(env)
   user.id if user