Menu doesn't need to worry about box-sizing

We used to set the menu width by hand, but since we don't do that
anymore, we can probably get by without all the box-sizing.
Remove most borders

We'll keep a simple 1px border around the dropdown menus in desktop
mode, but everything else feels pretty theme-specific.
Use Grunt to compile LESS

This also moves the compiled file from the styles folder to a css
folder.
Add prefixes automatically
Minify using Grunt