Allow absolute links to css and js files (for CDN links)

Created by: Treer

I made this change because I was going to use CDN resources (Google fonts I think) in a theme. I'm no longer working on that theme, but someone else might want similiar functionality one day.

Absolute URLs/CDNs would mean files already in the cache when you browse to a new hub, but I assume it's much preferable for "stock" Hubzilla to avoid any dependencies on outside services, so I expect this change is low priority.

It's low priority for me too - I'm commiting it as otherwise it'll get buried in the stash stack, plus this way I'll find out if CDN themes are out of the question (perhaps the project has a security ideas about not fetching outside urls).

Only other difference is that script_path() . '/' now gets evaluated even if $path isn't set.