Make learning mastery gradebook more accessible

Fixes: CNVS-13520

Add label to "student learning master gradebook" "outcome group"
expand all and collapse all buttons, and
cause them to properly set the aria-expanded attribute on each
individual item. Also disable button when they are irrelevent.

Test Plan:
1) Create a course, add a student, and turn on the student learning
   mastery gradebook
2) Add at least two outcome groups, each with at least two outcomes
3) Assume the identity of the student, and visit the gradebook,
   learning mastery tab
4) With your screenreader, verify that the outcome group exapand and
   collapse all buttons are usable.
  * Validate that the Expand All button is reported as disabled for
    screen reader users if and only if all items are collapsed,
    regardless of whether they were expanded via the expand all button,
    or manually, one-at-a-time.
  * Verify the inverse of the previous for the Collapse All button.

Change-Id: I685dab01f93608c5a34204bf3403d3f6fd4aa793
Reviewed-on: https://gerrit.instructure.com/37927
Reviewed-by: Aaron Cannon <acannon@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: David Josse <david@instructure.com>