Add support for anchors in css cachebust urls.

* Font urls in css will often have an anchor which was causing
 "not found" errors for those files during cachebust.
* When using a domain relative url (one that starts with a /)
 the file can't be found. Change to just append the url to
 the project root for a higher likelyhood of finding the file.

Point 2 works well for bedrock anyway :)

I looked into writing tests, but no tests exist for any of the
cachebusting code, or any of compress_assets for that matter. I'll
try to get some good tests of this code going in a subsequent PR.