Support a configurable Rails asset_host to allow for CDNs

Adds support for https://gitlab.com/gitlab-com/infrastructure/issues/57

config/environments/production.rb

@@ -50,7 +50,7 @@ Rails.application.configure do
   # config.logger = ActiveSupport::TaggedLogging.new(SyslogLogger.new)
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server
-  # config.action_controller.asset_host = "http://assets.example.com"
+  config.action_controller.asset_host = ENV['CDN_HOST'] if ENV['CDN_HOST'].present?
 
   # Precompile additional assets (application.js, application.css, and all non-JS/CSS are already added)
   # config.assets.precompile += %w( search.js )