Make the back button work for the Gallery with infinite scroll

[jmathai]

If you scroll past many pages, navigate forward and hit the back button you're taken to the beginning of the collection.

One proposed solution is to have a start/end page parameter and use pushState to keep track of the pages which have been loaded.

The drawback here is that if you scroll to page N then the back button may take you to page N-1. I think that's not ideal. So we can possibly push the state silently with Backbone.

Either way, we should define what the behavior ought to be when you scroll to page N and hit the back button.