Fix z-index issues for the AJAX spinner and headbar menus.
The z-index for the AJAX spinner caused it to appear below the review

dialog, which looked a bit wrong. It's now set to be higher.
The user and support menus were also appearing above everything else,

due to an attempt at fixing a z-index issue in the admin UI with grid

filters. Turns out we had a rule for the filters to set the right

z-index, but it was being overridden. We now force it, and no longer

have the custom z-index.

Tested all the broken z-indexes in relation to each other. Things look

correct now.