Add indexes to message.subject and thread.subject.

Summary:
Otherwise API queries that filter by ?subject=<...> get slow. This also skews
our response time metrics, since the monitoring does a bunch of these queries.

Test Plan: Nada.

Reviewers: khamidou, spang

Reviewed By: spang

Subscribers: spang

Differential Revision: https://review.inboxapp.com/D531