Take quiz: accessible timer!

So cool! Press ALT+SHIFT+T (or ALT+CTRL+T) at any time while you're
taking the quiz, and screen-reader will read the time remaining right
away without stealing focus away.

Closes CNVS-14086

TEST PLAN
---- ----

  - make a quiz with a time limit
  - take it and turn on VoiceOver or something
  - right after the quiz instructions, you should now hear a new note
    explaining the new shortcut
  - now press ALT+SHIFT+T
    + verify you hear the remaining time
    + verify your cursor is still where it is; you can resume doing
      whatever you wanted to do after hearing the remaining time

Change-Id: I18ccce6bf2f85c1f60206a61a46b8ef683a79dea
Reviewed-on: https://gerrit.instructure.com/41680
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Taylor <rtaylor@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Derek DeVries <ddevries@instructure.com>