Improve scrolling behavior for lightboxes in Safari

Summary: Our "html { overflow-y: scroll; }" makes Safari flip out when we put "hidden" on body. Instead, put the scroll on body and then replace it with hidden when the lightbox is visible.

Test Plan: In Safari, the body scrollbar vanishes when the lightbox is active and scrolling no longer causes spasms.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D3800