Respect course soft conclusion date for quiz subs

Submissions will no longer be creatable if the course has been deemed
concluded (softly; in the sense that its `end_at` overrides the term's
`end_at`, or its term has ended).

In this scenario, we now display a message to students (and teachers):

  "This quiz is no longer available as the course had been concluded."

Closes CNVS-15934

TEST PLAN
---- ----

  - create a course and open its settings page
  - set an end date to sometime in the past
    + do NOT check the option "Users can only participate in the course
      between these dates" yet
  - create a quiz in that course, with like a single question and
    unlimited attempts, and then publish it
  - in another tab as a student, go to the quiz take page, and take it
    once, hand it in, then stay on that page where you can see "Take
    this Quiz Again"
  - now in the teacher tab, edit the course settings and turn that
    option on, save
  - go back to the student tab, do not refrehs, just click that button
    for taking the quiz... and:
    + verify you are redirected back to the same page (e.g, not taken to
      the take quiz page)
    + verify no new attempts have been registered
    + verify you now see the alert message in a red box
  - turn that option back off, refresh the page:
    + you should be able to re-take the quiz
  - try on and off, see if you can get anything funny going on

Change-Id: I83a373258cc7e615c98e416ef7d630fa9ccb7331
Reviewed-on: https://gerrit.instructure.com/43204
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: Hilary Scharton <hilary@instructure.com>