Sort out a more mobile-friendly navigation menu

[zarino]

Even on a fairly large phone screen, the nav menu takes up most of the initial viewport!

We should hide the nav menu on narrow screens if javascript is available, and then allow mobile users to show/hide the nav menu with a toggle button.

Bulma has a Nav component, but I it’s probably not going to work in our situation, and it’s no more responsive than what we’ve got at the moment anyway, so I think we should just homebrew a toggle button.