Fix circular import

Running the `create_oa_submissions` management command leads to a stack trace due to a circular dependency: https://gist.github.com/feanil/570f733c81803cfbb7a1

As a short-term fix to unblock the load test, I've moved the imports into the functions that use them (in workflow).  Longer-term, I suspect we'll need to more carefully define the boundaries between the `assessment` and `workflow`.

@ormsbee @stephensanchez