Caused discussion settings headings to be recognized as such by screen readers.

Fixes CNVS-6642

Test plan:
* With a screen reader, visit the discussions list page.
* Open the discussions' settings dialog.
* Verify that the "My Settings" and "Student Settings" headings are read as
  headings by the screen reader.

Change-Id: Iae8723452d5d8a5dd5db6e2548673a9eaa6a9fe9
Reviewed-on: https://gerrit.instructure.com/32396
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>