Allow lightbox comments to be viewed logged out

Summary: Fixes T12160. Lightbox thread view should be visible if file is public.

Test Plan:
Add a file to a task, log out, click on file in task, get lightbox and no error. Expand comments, see login box.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12160

Differential Revision: https://secure.phabricator.com/D17347