Mobile Menu Support

This is just a first pass at adding accessible menus into the 4.x Prototype branch. It is ready for community feedback & additional testing.

This looks great!

The only change I am looking for is to clean it up a little bit:

1. Add aria-label to the <nav>
2. Remove the aria-labelledby on the <nav>
3. Remove the visually hidden h2.

This way it is just a little cleaner:

<nav aria-label="main" id="block-mainnavigation">

Thanks @michaelablackham for your feedback and testing! This has been merged into the 4.x branch and will be included in the next release.