Accessibility: Enclose discussion titles in h3s

Fixes CNVS-14659

Test plan:
- Visit the list of discussion topics for a course.
- Verify that the discussion titles are recognized as level three headings by
  screen readers.
- Verify that the visual appearance of the page has not been effected.

Change-Id: Ideddc504730cd0e8cdc91a8f34174922acafed76
Reviewed-on: https://gerrit.instructure.com/38886
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matthew Wheeler <mwheeler@instructure.com>