When creating a draft from a review request, the extra_data instance was

being assigned to the draft, instead of being copied over. This meant

that if any code tried to write into the draft's extra_data, it would

also appear in the review request's.
This was first noticed when attempting to do an API POST to the draft

resource that both changed an extra_data field on the draft and

published in one go. The publish code thought that the data hadn't been

changed, since it was in the shared extra_data, and would complain.
We now do a deepcopy of extra_data to prevent any sharing issues.

Added a new unit test that failed with the old behavior. It now passes.
All other unit tests pass too.