fix polymorphic quiz_submission association on delayed_message

fixes CNVS-12395

test plan:
  please check for regressions around notifications on quiz submissions.
  this includes:
  - to teachers when the submission needs grading
  - to students when the submission is graded
  - to students when the grade changed

this one is very difficult to reproduce manually. data needs to be in a state
where a delayed job has been queued to send a notification message for a quiz
submission before the namespace change was deployed, and then it attempts to
execute the delayed job after the namespace change. it appears that messages
are only sent to a delayed message queue when there there are more than 500
messages per day.

Change-Id: I0869b4decaec900a2c4c8df5c6e0b8b29dd08c5c
Reviewed-on: https://gerrit.instructure.com/33307
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>