accessibility for what if score

fixes CNVS-7088

test plan:
  - Do the following with OSX screenreader and JAWS
    - as a student go to the grade summary page
    - using screenreader controls navigate to the what-if score
    - change the score and revert the score back to what it was
    - navigation should work properly
    - text that is read aloud should make sense
  - note: after changing the score you will refocus on the revert link,
    this is a fix for an OSX screenreader bug (and hopefully helpful too)

Change-Id: I84482d203e5ec73a4bc374bb663a0d451c502d51
Reviewed-on: https://gerrit.instructure.com/27995
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>