CQS - roundhouse a11y powerups

Closes CNVS-16313, CNVS-16314, CNVS-16315

Here are the items covered by this patch:

- Discrimation Index help dialog will now auto-focus the close button
  instead of the first paragraph
- Toggle Question Details buttons are now hidden from the SR (both the
  individual question buttons, and the master one)
- Wording of the audible table summary for the answer distribution chart
  has been modified, see below for the new text.
- Bracket answer ratios in the DI table are now simplified; if we have
  60% correct answers in that bracket, it will no longer speak "and 40%
  got them wrong" as it was redundant
- Answer "drilldown" (aka question details) are now hidden from SR as
  that data was already covered by the tabular chart alternatives

TEST PLAN
---- ----

  - go to the new stats page and verify the items above are in place
  - (optional) take a look at the affected tickets, there's some
    discussion there that could give you some context

Change-Id: Ia34540f2b20c1387a83b5718e6884c9092e641f4
Reviewed-on: https://gerrit.instructure.com/43562
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Reviewed-by: Aaron Cannon <acannon@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>