don't set graded_at in the future for quiz submissions

fixes CNVS-14403

test plan:
- create a quiz with a due date far in the future (and no time limit)
- take the quiz as a student
- look at the grade_history api for that user
- the graded_at for the quiz assignment should be when it was submitted, not
  when it was due (in other words, it should be in the recent past, not the far
  future)

Change-Id: I37754b4be5238e240bbc6a9826ea9d7599c9a61f
Reviewed-on: https://gerrit.instructure.com/38154
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>