R1:80de8c93c9db

Stabilize Thread-Topic

Summary: NOTE: This can break current ongoing conversations.

Test Plan: Commented on a revision and checked the header in the e-mail.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1340

Differential Revision: https://secure.phabricator.com/D2723