speed up GradebooksController#update_submission

closes CNVS-12306

Test plan:
  * updating grades in the gradebook(s) should still work (including
    setting default grades and gradebook history).
  * updating grades/commenting in speed grader should also still work

Change-Id: Ia71e39ea2a630f7732a68c5023d8a4b4b97a7ae4
Reviewed-on: https://gerrit.instructure.com/29120
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>