DA - quiz dates for observers

fixes CNVS-15954

test plan:
  - as a teacher make some differentiated quizzes
  - go to the quiz index, it should work
  - as an observer with no students
    - go to the quiz index
    - quizzes dates should load (no spinning arrows)
      - (if they dont have a date thats fine, they
        just cant be stuck loading)
  - as an observer with students
    - go to the quiz index page
    - it should load the quizzes your student(s)
      can see

Change-Id: Ieeaac8c986afff37724513121113cbdd37fd47d0
Reviewed-on: https://gerrit.instructure.com/42098
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>