Adam/generalized draft import export 2

@dmitchell -- is this the right idea?

https://openedx.atlassian.net/browse/TNL-574
xml backed courses can only use deprecated strings. idk if this still really creates a deprecated string or now just gives the 'right' string for the key which would be the wrong string for xml. Arggh. I would expect there to be a reference conversion fn someplace around here which should do the "right" thing.