Mobile Safari doesn't place modals or notifications at the highest z-index

[gakimball]

Mobile Safari on iOS 7 and 8 doesn't seem to want to take modals or notifications, which have the highest z-index on the page, and make them appear above everything else. In our documentation, modals were appearing underneath the nav bar—we got around it by moving the modal directives all the way out to the root of the HTML.

It has something to do with how Mobile Safari handles z-index with fixed-positioned elements. Which is to say, not well.