add a private API endpoint for getting quiz submission HTML

Test plan:
  - As a teacher and student, the quiz show page should show the quiz
    show page as usual with submission results at the bottom.

closes CNVS-12452

Change-Id: I5844c9594a769dd1d5204db9a7e841e23b789382
Reviewed-on: https://gerrit.instructure.com/33509
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Stanley Stuart <stanley@instructure.com>