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
