Delete accounts

Summary:
Christine, Charles, Eben

I've implemented the minimum viable code for deleting accounts. There's not a lot of changes in this differential;  mostly I changed foreign keys definitions where needed. I also extended session_scope() to autocommit when needed.

One thing I'd like you to review in priority is the deletion logic. Right now it consists in stopping the account syncing, waiting 60 secs and and then marking the account as deleted.

This is needed because otherwise inbox-start may crash. I think it's okay to wait like this, though one minute is a little optimistic. What do you think about it?

Finally, I added a small script, `garbage-collect-db`, which removes accounts marked as deleted older than ten days.

Ps: migrations are coming in the next version of the review.

Test Plan: Ran a test which deleted accounts at teardown. Checked that the accounts were deleted.

Reviewers: spang, charles, emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D351