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.
Importing `upload_backend` directly means that in testing it must also
be mocked where it is imported, in order to correctly test the right
backend. Since `get_avatar_url` is part of the public
`ZulipUploadBackend` API, add another helper method to call that.
Alex Vandiver