Added headings to make discussions easier to navigate to for screen readers

Fixes CNVS-11968

Test plan:
* visit the discussions page.
* Validate that the visual appearance hasn't been negatively impacted.
* Validate that the names of discussions are read as headings by screen
  readers.
* Validate that graded discussions are indicated as such by screen readers

Change-Id: I758bb951a4fe372e9d79313505825e375857ec4a
Reviewed-on: https://gerrit.instructure.com/32243
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>