Fix URL building for nested resources

`active_admin_config` only works for the current resource being viewed
and won't work for associated resources. This commit fixes that to use
the active_admin_namespace to look up the resource definition for the
given resource.

lib/active_admin/reorderable/table_methods.rb

@@ -11,7 +11,11 @@ def reorder_column
       private
 
       def reorder_handle_for(resource)
-        url = [active_admin_config.route_instance_path(resource), :reorder].join('/')
+        url = [
+          active_admin_namespace.resource_for(resource.class).route_instance_path(resource),
+          :reorder
+        ].join('/')
+
         span(reorder_handle_content, :class => 'reorder-handle', 'data-reorder-url' => url)
       end