Prevent double lightbox in lightbox comments

Summary: Ref T3612, prevents lightbox from spawning from inside a lightbox.

Test Plan: Click on file lightbox, leave file comment, click file comment, get take to file page instead of another lightbox.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3612

Differential Revision: https://secure.phabricator.com/D16978