R1:ddb1f181c1be

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…