Ember Quiz Statistics - Summary section

Closes CNVS-11697

TEST PLAN
---- ----

  - create a quiz with a number of questions, you can use multiple
    choice or T/F exclusively for this patch
  - take the quiz by like 5-10 students
  - visit the statistics page
  - verify the following items have proper values:
    - avg score
    - high score
    - low score
    - std. deviation
    - avg time taken
  - verify that the trend line/median of score distribution line graph
    is visually correct
  - verify that the score percentile column chart is correct
  - try generating student and item analysis:
    - verify you see a progress bar that updates as the thing gets
      generated
    - verify that hte tooltip content changes to reflect the states of
      the report and/or its generation
    - verify that it auto-downloads the report when it's generated
    - verify that once its generated, you can click the link to download
      the file
    - extra: click generate, refresh before the report is generated, and
      verify that the auto-download no longer occurs
    - verify that you can't spam the generate button; it will generate
      the report only once and prompt you to download only once (it
      should become gray/disabled once you start generating)

Please see the following document for the AC I'm working with alongside
the JIRA ticket, and for the status of some of the items:

http://docs.amireh.net/canvas/cnvs-4338#Summary-section

Change-Id: I2348e21daf6e33cc9af3746a445aa192a371fb35
Reviewed-on: https://gerrit.instructure.com/32733
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>