[SCHEMA] Sync Gmail label deletes.

I know cascades are scary, but I don't see how we could execute this
code with bad data from the remote, and no message objects are ever
deleted by it, just labels on threads and imapuid entries. These deletes
will be synced later anyway, and this reduces our workload and the
complexity of the syncing code.
Gmail/IMAP: sync folder/label names on every poll.