only send notifications the first time for a quiz regrade

test plan:
  - Create two students in a course.
  - As one of the students, complete a quiz. In a different browser
    window, start taking the quiz as the other student.
  - As a teacher, in a diffrent browser than the student taking the
    quiz, regrade the quiz.
  - You should get a "Quiz Regrade Notification".
  - Now as the student who is taking the quiz, submit the quiz. Their
    quiz should get regraded and you should NOT get another Quiz Regrade
    Notification.

fixes CNVS-9485, CNVS-8358

Change-Id: I20d254e32001e0932f17a906703675589c333d35
Reviewed-on: https://gerrit.instructure.com/28955
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>