Accessibility: heading levels reflect discussion reply nesting

Fixes CNVS-12502

Test plan:
- Create a discussion with threaded replies.
- Using a screen reader, verify that replies with varying levels of depth
  have varying heading levels to match.

Change-Id: I85fc47d54ef86e4ddcc0c5bdb184ec483e649b03
Reviewed-on: https://gerrit.instructure.com/35770
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>