Hide empty shopping cart.

This doesn't work perfectly. An empty cart is hidden, but when you add
the first item, it reveals an empty cart. A 2nd item added does show two
items. There's also a window flash between some actions.

But right now, I'm not worried about Ajax. I just want to keep moving
forward to the stuff I'll need to use sooner.
Merge branch 'hide_empty_cart' into develop