Bug/import

Publish was breaking if the were children references to non-existent items. Because import uses publish, this was breaking imports of some courses.

Import was breaking if a tar.gz had a directory in it w/ the same name as the root dir which occurred importing some MITx courses with manifest files.

@ormsbee @cpennington plz review
I made it a kwarg like on update_item for allow_not_found and have the xml_importer setting the kwarg