Revert "Allow notifications to sit over the nav bar"

Reverts TryGhost/Ghost#4544

Something I sadly didn't see in testing #4544, was settings menus showed over the content, until the transition stopped. It's ugly and needs to go. Unfortunately, it's not a super quick fix and needs revisiting.

[Here's the issue](http://cl.ly/0e1I062p3A3w) < 500kb video