Previously, api.update_from_assessments() would call model.
update_from_assessments(), then api._serialized_with_details(reqs), which
would duplicate the update work we *just* did as part of getting the
status. Now, update() and get_details() are totally seperate operations, and
_serialized_with_details() is a read-only method that does not need to have
requirements passed in.

I've also standardized student_training and peer, the two assessment types that
do *not* auto-complete on None requirements, to return False if None, and error
out if passed {}. This is to allow staff_assess to pass None to indicate
"Update thyself, if you don't depend on requirements" to the other step types.

Also adding test_requirements_changed(), to confirm that workflows are updated
when requirements change, even though thoe requirements are no longer explicitly
passed to the getter.