Simplify lightboxing and fix a few things

Summary:
Minor tweaks to lightboxes.

• With "position: fixed;", we don't need to do any of the scroll/resize stuff. Just remove it.
• Make the lightbox go over the menu bar -- was it intentional that it wasn't?
• Make 'jx-mask' use "position: fixed;" too.
• Add a loading indicator.
• In Differential/Maniphest/etc, a preview may bring in an image but won't bring in the CSS we need. The "real" fix is to ship CSS/JS with ajax, but that's really hard -- fake it by pulling in the right CSS any time we render a remarkup area.

I'm going to do a couple of other tweaks here but need to update JX.Mask.

Test Plan: Verified behavior is reasonable in Safari, Firefox, Chrome with multiple images / scroll / previews / resize.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1896

Differential Revision: https://secure.phabricator.com/D3795