Fix logging for tests.

logging.fileHandler will create a file, but not a directory, for you, so we
need to make sure the directory (now just /tmp/log) exists.
Delete empty threads on draft deletion.

Summary: Otherwise we end up with empty dangling threads.

Test Plan: Regression test added.

Reviewers: kav-ya

Differential Revision: https://review.inboxapp.com/D910