Improve navbar UX logic for #233

This change takes us away from using overflow:hidden on body by
default, only toggling this using a new `open` class when the navbar
has been expanded. `open` is toggled on body in the same places as it
is for the appear and navdrawer.
Address feedback on #238
Merge pull request #238 from google/fix-233

Improve navbar UX logic for #233