Add a `snippet` field to the API response for threads.

Summary:
Currently it's just the snippet from the most recent message in the thread.

Also fix a typo in which we thought we were ordering Thread.messages by message
received date, but actually weren't.

Test Plan: Manual testing.

Reviewers: mg

Reviewed By: mg

CC: bengotow

Differential Revision: https://review.inboxapp.com/D133