allow the quiz submission users endpoint to sideload quiz submissions

Test plan:
  - As a teacher in a course, create a quiz.
  - Have some students take the quiz, and some students not take the
    quiz.
  - Query
    '/api/v1/courses/:course_id/quizzes/:quiz_id/submission_users?include[]=quiz_submissions'
  - You should see quiz submission data in "quiz_submissions" in the
    json root with the quiz submissions for each user.

fixes CNVS-12896

Change-Id: Ia5ca15c07c97c009267fc52dcd02c61355fcfd2f
Reviewed-on: https://gerrit.instructure.com/34422
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>