Use add_columns instead of add_entity

This is a step towards converting the _get_nested_collection_attributes
method from Query to Select. The add_entity method does not exist on
Select; add_column should work the same way, in theory...

lib/galaxy/model/__init__.py

@@ -6184,7 +6184,7 @@ def attribute_columns(column_collection, attributes, nesting_level=None):
             .add_columns(*attribute_columns(DatasetPermissions, dataset_permission_attributes))
         )
         for entity in return_entities:
-            q = q.add_entity(entity)
+            q = q.add_columns(entity)
             if entity == DatasetCollectionElement:
                 q = q.filter(entity.id == dce.c.id)
         return q.distinct().order_by(*order_by_columns)