CQS: DI help dialog toggle button a11y

This patch makes the button that opens the discrimination index help
dialog more accessible.

Closes CNVS-15306

Enhancements:

  - the "?" little icon now acts (and reads) like a button
  - button is now reachable via TAB
  - button now reads a descriptive message when the user focuses it:
    "Learn more about the Discrimination Index"
  - pressing RETURN while focusing the button will open the dialog
  - the SR starts reading the content of the dialog as soon as it
    is opened, instead of focusing the Close button or the link inside
  - the link used to read "here", now it contains the full message:
    "More information is available here." which is better

TEST PLAN
---- ----

  - go to statistics cqs with at least 1 multiple choice question
  - keep pressing TAB and verify you can focus the "?" icon
  - verify that you hear "Learn more ..." when you focus it
  - press RETURN
    + verify the dialog opens
    + the thing should start reading the little article in there
    + verify you can TAB to the "More info..." link in the article
      - click it and verify it takes you to the instructure help article
      - close the tab
    + press ESCAPE and verify the dialog closes, and your focus is back
      to the toggle button

Change-Id: I1b840ed82b9d71262c92372aeb7182ef76707fc9
Reviewed-on: https://gerrit.instructure.com/40779
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>