Adam/generalized draft import export 2

@dmitchell -- is this the right idea?

https://openedx.atlassian.net/browse/TNL-574
yeah, now it's for any "root" draft module: https://github.com/edx/edx-platform/blob/adam/generalized-draft-import-export-2/common/lib/xmodule/xmodule/modulestore/xml_exporter.py#L155