Improve rendering of new-style Differential comments in feed/notifications

Summary: Ref T2222. This probably doesn't get everything, but should improve many of the newer transactions.

Test Plan: Looked at feed after making some edits.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8368