apps.core.models.assignment_group: Added close_groups() to the manager.
devilry_examiner...close_groups: Cleanup the layout.
devilry_examiner...close_groups: Use the new close_groups method on

the AssignmentGroup manager.
devilry_examiner...test_close_groups: Test that delivery_status is set correctly.
apps.core.models.assignment_group: Added can_be_given_another_chance property.
devilry_examiner: Tuned translation.
devilry_examiner: Show correct messages on closed-without-feedback.