CQS: Feature Flag & Ember quizzes purging

- The "New Quiz Statistics" feature flag now converts the regular
  statistics page into the new one that uses the React/CQS client app
- All Ember quizzes code gone
- canvas_ember_urls gem gone
- quiz_moderate feature flag gone
- quiz_stats feature flag is now allowed in prod
- statistics_cqs explicit/hidden route gone, but the statistics_cqs ERB
  and coffee bundle are still there to help ease the transition later

Closes CNVS-16330

TEST PLAN
---- ----

  - go to /courses/:course_id/quizzes/:quiz_id/statistics
    + verify you get the old/regular stats page
  - enable the "New Quiz Statistics" feature flag
    + refresh that page, verify you see the new app

Now turn the flag on and off and play around the quizzes pages, verify
everything looks OK.

Change-Id: I01fc5da80fd206df08338823ffb446a1e3a06ed0
Reviewed-on: https://gerrit.instructure.com/42965
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>