Make Thread-Topic human readable

Summary:
Some e-mail clients display this header and it needs to be constant.

This is somehow involved but I doubt that there is a simpler solution.

Test Plan:
Applied SQL patch.
Commented on revision, commented on commit, changed package.
Verified that the Thread-Topic has constant and human readable value.

Reviewers: epriestley

Reviewed By: epriestley

CC: ola, aran, Korvin

Differential Revision: https://secure.phabricator.com/D2745