utils: Add path_to_url() and generalize get_relative_path()

The old get_relative_path() implementation assumed os.sep == '/',
which doesn't hold on MS Windows.  The new implementation uses
split_all() for a more general component count.

I added path_to_url(), because the:

  '/'.join(split_all(path))

idiom was showing up in a number of cases, and it's easier to
understand what's going on when that reads:

  path_to_url(path)

This will fix a number of places where I think paths and URLs were
conflated, and should improve MS Windows support.