CQS: answer drilldown accessibility

This patch provides two a11y enhancements:

  - question statistics now have an invisible h4 that reads the question
    position, so the listening user can tell which question we're
    actually talking about
  - when you click the "Toggle Details" button, the new state of the
    button is read automatically by the SR

Closes CNVS-15305

TEST PLAN
---- ----

  - for a quiz with at least 1 question, go to statistics CQS page
  - search for headings using the screen reader and verify that each
    question box has a heading that reads "Question X" where X is the
    position of the question
  - tab through to the "Toggle Details" button for a question and press
    it
    + verify you hear "Hide answer details"
    + press it again
      - verify you hear "Show answer details" again

Change-Id: I85ddcc6219c0022e7979d06ba224de856575076f
Reviewed-on: https://gerrit.instructure.com/40777
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>