Rewrite asset URLs if ENV[asset_cloudfront] == true to prevent stales

* if image assets get referenced in CSS, they get converted to
  something like:
  * https://abcd1234.cloudfront.net/images/sample.png?1234567890
* To prevent stale assets from being served from Cloudfront:
  * https://abcd1234.cloudfront.net/a/images/sample.png
* We indicate if cloudfront is being used with the environment var
  "asset_cloudfront", which is set to "true"