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
