CQS: accessible discrimination index

Closes CNVS-15304

Enhancements:

  - "Discrimination Index" label that contains the number now reads as
    a full sentence
  - chart is now ignored by SR
  - chart is substituted by a table that provides a friendly sentence
    representing the DI for each bracket (more on this below)
  - the table is equipped with a nice description of what the table is
    about, and what the student brackets are

Other relevant enhancements:

  - in the summary table, when you go to the Average Time column, doing
    a read-sentence will no longer speak the "mm:ss" timestamp along
    with the friendly audible version, only the latter (tested on VO,
    needs checking on JAWS/NVDA)

Here's how the table should be read for DI data like this:

[ Top bracket | 100% correct ]
[ Mid bracket | 0%   correct ]
[ Bot bracket | 30%  correct ]

- "Top bracket: all students in this bracket have answered correctly."
- "Middle bracket: not a single student in this bracket has provided a
  correct answer."
- "Bottom bracket: 30% of students in this bracket have answered
  correctly, and 70% have not."

TEST PLAN
---- ----

  - go to CQS page, use a screen-reader
  - locate a multiple-choice or true/false question with some DI data
  - go to the DI chart
    + verify the enhancements above are in effect

Please test this under NVDA and JAWS as well as VO, Hopefully the
inconsistencies won't be that bad, but I have no idea.

CR NOTES
-- -----

I've come up with a decent hack for forcing VO to not read "sighted"
user content by exploiting the [hidden] attribute and some CSS. It
shouldn't have any regressions as it's an opt-in hack, and has a very
nice benefit for Say-All and Speak-Sentence modes.

Just so you know when you're looking at the CSS and SUC component
changes.

Change-Id: I986f1566f4b44b200b2bb89e0b9432190841d397
Reviewed-on: https://gerrit.instructure.com/41332
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
Product-Review: Ahmad Amireh <ahmad@instructure.com>