add feature flags for quiz statistics and moderate pages

fixes CNVS-13228

the old way of enabling fabulous quizzes is no more. Now we're using proper
course based feature flags, and we have two different feature flags for
the new statistics page and moderate pages. You can enable these flags from
your account settings page.

In the code we now check these using:
  @context.feature_enabled?(:quiz_stats)
  @context.feature_enabled?(:quiz_moderate)

In ember we have a new environment attribute to check if moderate is enabled:
  env.get("moderateEnabled")

This also moves "take quiz" to the top item in the list, and changes its icon
to be a play icon to not have so many circles as icons in the menu

test plan:
  - feature flag changes
    - as a site admin
      - disable both quiz stats and quiz moderate
      - visit the quiz index/show/moderate/statistics pages
      - they should all be the non-ember versions of the pages

     - enable the quiz stats feature
       - as a teacher
         - visit the quiz index/show/statistics pages
         - this should be the ember version of these pages
         - visit a quiz page
         - this should enable the ember version of these pages
         - there should be a link to moderate the quiz in the dropdown menu
         - try to visit the moderate page (quizzes/123/moderate)
         - this page should render correctly as the old moderate page
         - the moderate tab should not be there

    - enable the quiz moderate feature
      - as a teacher
        - visit the quiz page
        - the moderate link in the drop down menu should be gone
        - this should enable the moderate tab in ember
        - visit the quiz moderate page, this should be the ember version
        - if you try to visit quizzes/123/moderate directly it should
          redirect you to the moderate page

  - api changes
    - as a teacher
      - request the quizzes api endpoint for a single quiz
      - you should see a new "moderate_url" property in the payload

    - as a student
      - request the quizzes api endpoint for a single quiz
      - you should NOT see the "moderate_url" property in the payload

  - other changes
    - with stats enables, the take quiz should be at the top of the dropdown
    - the take quiz icon has been changed

Change-Id: I04639890b425b1c5abf3305892e4b5b9d804b525
Reviewed-on: https://gerrit.instructure.com/35810
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>