Getting draft split_test_module instances to import/export.

STUD-327

@cpennington and @nasthagiri I am working on getting draft instances of split_test_module to import/export correctly. I corrected some issues with assumptions about draft verticals being nested immediately below sequentials. However, after fixing those issues I ran into another one that I don't know how to fix.

I put comments in the code below explaining the problem that I'm seeing. Basically, when we try to rewrite the user_partition_id -> vertical mapping for each group, the rewrite fails because the translation from deprecated Location string to UsageKey ends up with the wrong course run. I can't figure out where/how to fix this. I am putting this PR up with copious comments in the hopes that one of you will have some ideas of how to get draft content to behave like published content.