Get nav menu classes working

This updates the classes in the LESS file, and fixes a place in the demo
that still used "section" instead of "navbar-menu-group".
Use more specific selectors in the demo

No need to modify styles for _all_ of the sections on the page.
Split out nav-label and make it mobile-first
Use a variable for nav height
Use new classes in CSS

Deeply nested indentation and child selectors are the worst.  Luckily, a
lot of that has been eliminated.
Update menu to use the new CSS classes