[fix bug 932866] Hamburger Nav Does Not Work FFOS

Sadly, Firefox OS doesn't play well when applying CSS transforms to `position: fixed;` elements. So for now we have decided to remove the transition animation on the mobile nav menu.

This PR also includes:

* 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 list.