Stub out back button events

Originally I thought we could tie these to the browser's back button, to help with development. But you can't actually catch the event when the Back button is pressed. We could listen to `onpopstate` and try to work out if it was a back action, but that would be hard to do agnostically, and we would never be able to implement `preventDefault` properly. So I figure a nonfunctional stub is better than a unexpectedly half-dsyfunctional implementation.