Improve Lightbox behavior for nonstandard clicks

Summary:
When users middle click or command-click an image, we should open it in a new tab, not open a lightbox.

See https://github.com/facebook/phabricator/issues/234

Test Plan: Left, middle, and command-clicked a lightbox image.

Reviewers: vrana, chad, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4020