Ajax commenting on lightbox images

Summary: Adds a comment box, you can put text into it, hit enter, and see it come back.

Test Plan: Put text into box, see it come back.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3612

Differential Revision: https://secure.phabricator.com/D16907