Allow for aboslute paths without full URL.

It's possible I don't 100% understand what you're trying to do here, but in order to get the URL mapping to work properly, I had to add the relative_url_root into the regex (I forgot that with the first pull request).  I also had to make the regex match whether or not the full URL was included.  This is because often paths come in like /assets/custom_ckeditor_config.js.  The first if doesn't prepend the basepath because it's absolute, but the regex doesn't catch it because it's
Escape any front slashes
Add relative url root to fall-back path creation.