Adam/generalized draft import export 2

@dmitchell -- is this the right idea?

https://openedx.atlassian.net/browse/TNL-574
I was unfamiliar w/ this pattern. Perhaps it's worth reminding the reader via a comment what's in `draft_parent_list` even tho it is only a few lines above here? "Pull the locations out of the (xblock, loc, parent) tuples."