Improved accessibility of showing/hiding and navigating to discussion sections

Fixes CNVS-11969 CNVS-11972 CNVS-11970

Test plan:
* using a screen reader, visit the discussions list page.
* Verify that the sections (Pinned Discussions, Discussions, and Closed for
  Comments) are headings.
* Verify that the buttons for toggling the visibility of each section are
  reported as buttons.
* Verify that the visual appearance of the page hasn't been negatively
  impacted.
* Insure that it is clear for a screen reader user what the toggle button does.

Change-Id: I620ccaf4b1c3e7b2f06076d84d9a15388a45d257
Reviewed-on: https://gerrit.instructure.com/32253
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>