Fixes #10433: Navbar z-index refactor

Specifically:

* Resets the default navbar z-index to auto at a certain breakpoint
* Adds fixed navbar z-index var to fixed bottom navbar (previously only on fixed top navbar)

Something to test out though: does the default `.navbar` class need to have a `z-index` set at all? The only way this matters is for fixed navbars, which get their `z-index`. Perhaps we can remove the initial declaration?