restore assessment request notifications

fixes CNVS-10485

test plan:
  * create an assignment with a rubric attached, auto-
    assigned peer reviews, and an online submission type;
  * as two students in the course with ASAP notification
    preferences, submit the assignment;
  * wait for the assignment's due date to pass, then verify
    that each student receives a reminder to peer review the
    assignment.

note: for the test steps to work, delayed jobs must
be running.

Change-Id: Ib066212df31e4af68fcff72a4de225b19db58077
Reviewed-on: https://gerrit.instructure.com/28586
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>