R1:e0cc277d5fbe

Fix mask positioning and use mask classes in lightbox

Summary: See D3795 / D3797. Also made the mask darker.

Test Plan: Mask now sizes properly on window resize in all browsers / mask uses.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3798