Bugfix: Number of deleted mappings not showing

Story: https://www.pivotaltracker.com/story/show/69081090

@bmomberger-reciprocity or @danring can you take a look at this one. The `binding` retrieved by calling `loader.attach(this.instance)` has an empty `binding.list`, which causes all the deferreds from this point on to be resolved immediately. Calling `refresh_instances` on the original binding (`self`) ensures the deferreds won't finish until the list items get refreshed, but I'm not sure this is the best solution for this.