[fix bug 932866] Hamburger Nav Does Not Work FFOS

Sadly, it seems Firefox OS doesn't play very well when applying CSS transforms to `position: fixed;` elements.

This PR includes:

* Removed the open/close transition on the mobile nav menu.
* Improved a11y, hiding the mobile nav from screen readers when it is not visible, as well as adding `role=navigation`.
* Focus is now set on the mobile nav once it is opened.
* Fixed an edge-case bug that allowed the page to scroll when `touchmove` even fired on the empty section below the open mobile nav menu.

Still awaiting confirmation on this direction in the bug, but this PR should at least be good for testing/feedback.