Make it so the diffusion comment panel can be haunted

Summary: ...basically by pounding the DOM a bit to be a little closer to differential. I also make the "add comment" UI show up if and only if the commit is rendering properly.

Test Plan: prezzed "Z" on diffusion and noted it was like differential now

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1177

Differential Revision: https://secure.phabricator.com/D3027