Ember Quiz Stats - DIndex fix

Discrimination Index bars were not uniform in their size which defeated
the point of the graph. This patch fixes that issue.

Closes CNVS-12937

TEST PLAN
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  - create a quiz with multiple choice and T/F questions
  - take it by a number of students and make the score chart diverse
  - visit the stats page and verify that:
    - the bars always have the same width total width, regardless of the
      score distribution
    - the bars always have the same width in all brackets
    - the bar's position sways to the left if the ratio of students in
      that bracket who got it *wrong* is higher than those who got it
      right
    - the bar's position sways to the right if the inverse is true

Change-Id: Ibfb149dbd29d6c0a73544b4465c40cf3e0c42f54
Reviewed-on: https://gerrit.instructure.com/34690
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>