Allow commenting on non-viewable lightbox files

Summary: Removes the viewable restriction on embedded files. Builds a basic lightbox UI for commenting.

Test Plan:
Add psd, pdf to Maniphest task, clicked on download, comment, left comment. Closed box.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3612

Differential Revision: https://secure.phabricator.com/D16917