Accessibility: Turned discussion edit button into button for SR users

Fixes CNVS-13971

Test plan:
- Visit a discussion with edit privileges
- Using a screen reader, verify that the edit button is reported as being a
  button

Change-Id: Ie89a3595d33f14fbb990dc2f8140216d5a08bfe2
Reviewed-on: https://gerrit.instructure.com/38469
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: David Josse <david@instructure.com>