Centralise url handling for urls to static resources

Centralise url handling for urls to static resources

Keep in CSSMin as-is for back-compat and to ensure library remains
independent of MediaWiki.

Moved down a few lines as there is no need to compute the md5 hash when we're
returning a data URI. Previously md5_file was called twice during module builds
(once for the fallback url, and another time when producing the embedded data uri).

Applied to logo in SkinModule as example. To be applied elsewhere as needed.
Without it, fallback is current behaviour (no cache invalidation).

Bug: T99096
Change-Id: I7f38bfc1bea5c241bc4f8ec4f4b640fd65f2c04f