send quiz_submissions url for jsonapi quiz api

Test plan:
  - As a teacher, create a quiz.
  - As the teacher, query the quiz api using the
    "Accept: application/vnd.api+json" header. You
    should see that "quiz_submissions" in "links" is null.
  - As a student, do the same query. "quiz_submissions" should be null
    again.
  - As the student, submit the quiz. Querying the API again, you should
    see a URL to the Quiz Submissions API for the user's quiz
    submission.
  - As the teacher, query the API again. You should see a link to
    the quiz submissions index API endpoint.

closes CNVS-11432

Change-Id: Ib2ee151c904b4800f811a85ea5142854ef8c3e3a
Reviewed-on: https://gerrit.instructure.com/30732
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Myller de Araujo <myller@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>