fix a few minor style issues in ember quiz stats

fixes CNVS-13608

test plan:
  - enable draft state and quiz stats
  - as a teacher
    - create a quiz with a multiple choice question
    - click the statistics tab
    - the blank slate image should have a little more padding at the top

  - as a student
    - take the quiz

  - as a teacher
    - refresh the statistics tab
    - the line in the summary should now be gone until we have a proper
      normal distribution curve
    - click on the (?) icon for discrimination index
    - the description in the help dialog should no longer have a scrollbar to
      view it all

Change-Id: I4f093bdfb206d8ba3f348f38c2a778ae278e1d73
Reviewed-on: https://gerrit.instructure.com/36389
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>