Fix missing select across logic in change_list

In the vanilla django admin, there's a blob that shows how many items you've selected, and exposes a button for doing a .all() on the queryset. This is currently missing in pops.

* [x] Needs design


![new old buttons](https://f.cloud.github.com/assets/189908/332180/37b49066-9c1a-11e2-8fa3-09335363ec17.jpg)