Add "phabricator-remarkup" to transaction comments

Summary: Fixes T2596. We currently omit this, so we don't get some styling (lists, e.g.)

Test Plan: Viewed a remarkup list in Macro, Pholio.

Reviewers: chad, vrana

Reviewed By: chad

CC: aran

Maniphest Tasks: T2596

Differential Revision: https://secure.phabricator.com/D5114