Ember Quiz Statistics - Base/Skeleton

A starting point for the implementation of Ember quiz statistics that
includes a route with the required data properly loaded, the necessary
serializers and adapters, and base stylesheet/template to start from.

This patch also adds a new submission statistic called
"submission_scores" that's basically a map between a score percentile
and the count of students who received it.

Closes CNVS-12171

TEST PLAN
---- ----

  - with fabulous quizzes on, go to a quiz show page
  - click the Statistics tab
    - verify that you see the blank page, and the tab is activated
    - verify that no errors are thrown in the console

Testing the new metric:

  - create a quiz with a certain number of points possible
  - take it by a number of students and score diversely
    - also let more than one student have the same score
  - perform an API request to retrieve the statistics and:
    - verify you get the new score distribution statistic and that it is
      correct
    - the metric should contain an entry for each distinct percentile
    - the metric entry should really reflect how many students got that
      score
  - check out the Quiz Statistics API docs and:
    - verify the new "scores" statistic under SubmissionStatistics is
      documented
    - verify that the documentation is clear enough

Change-Id: I1f00bd4c18a0767d6a50767c5d8868f1d6e561ac
Reviewed-on: https://gerrit.instructure.com/32732
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>