Quiz Stats - Set sanity limits

This patch makes the API reject generating stats for large quizzes. A
quiz is large based on two configurable settings:

  - quiz_statistics_max_questions (defaults to 100)
  - quiz_statistics_max_submissions (defaults to 1000)

They can be tuned using the console. Something similar to this:

    Setting.set('quiz_statistics_max_questions', 500)

The defaults can be changed by modifying constants in
Quizzes::QuizStatistics.

Closes CNVS-13330

TEST PLAN
---- ----

  - create a quiz with like 10 questions
  - change the setting for the max questions to something like 5
    - verify that you can't get the stats from the API
    - visit ember quiz stats:
      - verify you get a different page, similar to the blank state one
        but with a different message
  - tune it back up to something like 15
    - verify you can now get the stats from the API and can see them in
      ember

Change-Id: I1b0626c1b460974976eec5b184e1b5989f50d210
Reviewed-on: https://gerrit.instructure.com/35561
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>