[SCHEMA] Save read status on Message objects; support Gmail 'unread' syncback.

Summary:
 * Add an 'is_read' bit to Message objects, which just mirrors the IMAP
   '\Seen' flag.
 * Add support for syncing back unread status changes (at a per-thread level)
   for Gmail.
 * Also tighten the nullability constraints on 'UserTagItem.created_at' and
   'UserTagItem.updated_at'. This really should have been part of migration
   028; including it here for convenience.

Test Plan: Unit tests added.

Reviewers: spang, mg

Reviewed By: mg

CC: mg

Differential Revision: https://review.inboxapp.com/D150