Improved context for the unread and total replies numbers on discussions page

Fixes CNVS-11989

Test Plan:
* Visit the page that shows the list of discussion topics.
* Validate that the unread reply and total reply counts are read with context
  when using a screen reader.

Change-Id: Ib3ea7abc79a1a63c2f1d1d7e0c09049b20f6df6c
Reviewed-on: https://gerrit.instructure.com/32244
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <banderson@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>