Accessibility: Made expand/collapse controls accessible to keyboard users

Fixes CNVS-13584

test plan:
  - as both a teacher and a student:
    - navigate to assignments index page
      - using the keyboard verify:
        - you are able to navigate to the expand/collapse controls
        - expand/collapse appears visually focused
        - you are able to toggle the expland/collapse controls
    - enable :quiz_stats feature flag on a course
      - navigate to quiz index
      - verify the above for quiz index

Change-Id: Ia9486464289d122c41d1790202b1b5e81d1a67a8
Reviewed-on: https://gerrit.instructure.com/36785
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>