Adam/generalized draft import export 2

@dmitchell -- is this the right idea?

https://openedx.atlassian.net/browse/TNL-574
Nit: a couple of the lines above got changed after the comments for them were written. If you could take a second to update the comments (or just delete them), it would help the next person who tries to understand this test.

Line 223 should say # verify that the draft vertical also has a published version
I'm not sure what line 228 should say, but the comment doesn't make sense with the code below it.

I believe @dmitchell changed the helper method "get_and_verify_publish_state" so it returns True if a published version exists, even if there is also a draft version. 

BTW, I was trying to understand if you should have used get_and_verify_publish_state, and the answer is "no". The method doesn't seem to do what you would think it does.