Previously, tests that exercised code paths that added local
uploads did not always clean up `settings.LOCAL_UPLOADS_DIR`
after the test was complete.
Updates the `ZulipTestCase` class to remove any local uploads
in the unique `settings.LOCAL_UPLOADS_DIR` in `tearDown` for
all tests.
Actions like deleting realms may leave unreferenced uploads in the
attachment storage backend.
Fix these by walking the complete contents of the attachment storage
backend, and removing files which are no longer present in the
database. This may take quite some time, as it is necessarily O(n) in
the number of files uploaded to the system.
AcKindle3
Alex Vandiver
Lauryn Menard