[fix bug 1004204] Mobile navigation menu highlighting issue

This PR adds a focus-able heading when opening the mobile navigation menu, instead of focusing on the first menu item.

Focusing the first item in the navigation list is still causing confusion for some folks, as they associate it with highlighting the current page and not the focused state. This PR should hopefully address the main issue, while still making things accessible for screen readers. It also means we can leave our menu link focus states intact this way.

I've reused the `'Menu'` string already in the template for the heading, although we could use another that's more appropriate (and wait on l10n). I'm open to suggestions.

I've also taken the opportunity to remove of some linear gradient vendor prefix cruft from the mobile nav.