Changes include:
    -modifies requirements to gt an updated version of edx-submissions
    -adds set_staff_score to the AssessmentWorkflow model, which will record
        a new annotated score using edx-submissions functionality
    -prevents recording non-staff scores if a staff score exists
    -modifies update_from_assessment to call set_staff_score as needed
        -this includes changes to both the workflow model and its api
    -modifies get_score to allow staff overrides as optional
    -modifies the assessment serializer to include id information
        -adds this information to get_score method in self, ai, peer, and staff
            apis, to expose c ontributing_assessments where needed
    -fixes a small bug regarding None vs {} in the peer api
    -adds staff-assessment to the xblock, and makes it always available
    -uses the new force_update_score parameter on the workflow api when
        recording a staff assessment