update cached dashboard to-do list for grader after grading

test plan:
* with the rails cache enabled,
 create a course with an assignment and a student
* submit an assignment as the student
* as the teacher for the course, make sure the
 assignment shows up in the to-do list on the
 dashboard page
* verify that grading the submission updates the
 to-do list right away

* also verify that this works with quiz submissions

fixes #CNVS-2462

Change-Id: I39dff2cd4bac8a581c797e3e026ae23f981fc9c0
Reviewed-on: https://gerrit.instructure.com/39447
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>