touch all graders at once when saving a submission

fixes CNVS-15408

Change-Id: I5b3a9efeb6c38f849a34259442922dd279434ea5
Reviewed-on: https://gerrit.instructure.com/40887
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Nick Cloward <ncloward@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>