Hamburger menu for mobile

[neynah]

Related to #234

FWIW, the standard thing to do here is use a hamburger menu on mobile. Not that I like hamburger menus, but I think our strategy of dropping things from the top bar is unusual.

I agree with @kentonv that it's unusual to drop things from the nav bar. Our website is not currently designed to enable users to navigate easily from any page to any other page so dropping items from the nav bar only makes navigation more difficult for users. I'd like to design a hamburger menu that will display links to pages at widths where isn't sufficient room in the nav bar. I'd like to find a solution where we can have a hamburger menu and at the same time not lose all information scent.