Ember Quiz Stats - Summary chart scrolling fix

Closes CNVS-12672

TEST PLAN
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  - enable NQS, create a quiz, take it by one student
  - go to ember quiz stats:
    + verify that the summary chart shows the 0 and 100 percentiles
  - make the window small enough to show a horizontal scroller for the
    rest of the page
    + verify that the chart scales down to stay visible
  - make the window too small:
    + verify that the chart can be fully viewed using the horizontal
      scroller
  - make the window too large:
    + verify that the chart scales up to a certain point (width of
      960px)

Change-Id: I2be601aaa6c98f574fe6fa944a3a57558e386e3f
Reviewed-on: https://gerrit.instructure.com/38342
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>