Also fixes failing Python unit tests

Reset the correct URLs module to fix test failures.

Fix third party auth to reconfigure the registry on test cleanup.

Make the UrlResetMixin load the urlconf after resetting it, and fix the comment client test that was leaving ENABLE_DISCUSSION_SERVICE at True

Create a mixin class that can be used for tests that customize urls.py
to force django to reload it, so that they don't break other tests.