Allow routing with aliased admin registers

Now, if you have an admin panel set up like this:

```ruby
ActiveAdmin.register Catalog::Category, as: "Category" do
end
```

the sortable tables will still work. Basically we make no assumptions about what we think the route method is. We ask ActiveAdmin to just give it to us, and since we're working with member actions, we can just add the action name to the end of the route we get back.