Store labels in ImapUid

Summary:
This is a minimal patch to store label information per-folder. Note that this is only about storing data, not updating thread info using it.

I'd like you to take a look at the MutableList class I've added -- this SQLAlchemy thing is black magic.

There's one big question though: how do we update this info in prod? I was thinking we could have a one-off program that would go through every message of every account and query label information but this sounds super complicated to get right.

Test Plan: Ran a sync.

Reviewers: kav-ya, emfree

Reviewed By: emfree

Projects: #sync-engine

Differential Revision: https://review.inboxapp.com/D617