[SCHEMA] Add index on thread table for improved API performance.

Summary:
Diminishes API response times for /n/<ns_id>/threads?tag=inbox from >10 seconds
to ~780ms.

Test Plan:
Run migration script; benchmark against read replica of prod DB with
and without index.

Reviewers: khamidou

Reviewed By: khamidou

Differential Revision: https://review.inboxapp.com/D616