[fix bug 932866] Hamburger Nav Does Not Work FFOS

[alexgibson]

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.

[Osmose]

Menu works fine now on my Peak, and codewise I don't see anything wrong. You might want to ask Craig for a more detailed frontend review if you want, but IMO this is an r+.

Will merge once we get feedback on the bug (or merge yourself, or comment on this issue to get me to merge, anything is possible!).

[bensternthal]

Let's merge this one.