Simplify and fix static files serving.

Our urls were doing some backflips to be able to serve static files on
production environments when DEBUG=True, which conflicted with a change that
they made in django. We now serve static media only when DEBUG=True and
PRODUCTION=False, which means that we can use only
`django.contrib.staticfiles.views.serve`.

Testing done: Used the devserver.

Reviewed at https://reviews.reviewboard.org/r/5391/