Controller/model features for Outstanding Quiz Submissions

Outstanding quiz submission management is now contained in a dedictated
model, and this controller action provides access to the quiz scoped
actions of #search and #grade.  The API endpoint provides JSON access to
these actions.
Closes CNVS-15501
Test Plan:
  - Create quizzes with outstanding (unclosed, but overdue) quiz submissions
  - Hit the endpoint
  /api/v1/courses/:course_id/quizzes/:quiz_id/outstanding_quiz_submissions
  - GET JSON to retrieve quiz_submissions which are outstanding
  - POST JSON with something like {"quiz_submission_ids": [1,2,3,4]} to grade the QS
Change-Id: I7fe0341379d4dbd70ed25c667bd1c4dc7969b077
Reviewed-on: https://gerrit.instructure.com/41229
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ryan Taylor <rtaylor@instructure.com>