switch icons for quiz stats low score and std deviation

fixes CNVS-13620

test plan:
  - enable draft state and quiz stats
  - as a teacher
    - create a quiz

  - as a student
    - take the quiz

  - as a teacher
    - view the quiz stats page
    - in the summary section: low score should be a down arrow, stddev is a star

Change-Id: I1c0bbd57cab9316ed9f61e72c55e44527f25b171
Reviewed-on: https://gerrit.instructure.com/36418
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>