Fix issue with static files on Windows

[marcperrinoptel]

See #472
also https://code.djangoproject.com/ticket/34341

The probable takeaway from the discussion on Django side is that finders.find() is really supposed to be called with an OS-standardized path.

I've mimicked what Django does to an url before calling finders.find():

• step 1 (strip url base / prefix) was already done by whitenoise
• step 2 (url2pathname) is obvious enough
• step 3 (using posixpath.normpath, plus .lstrip('/')) seems debatable considering that we're already after url2pathname, but for the sake of mimicking Django I've included it

[Archmonger]

This needs a test to ensure there aren't regressions.