Discussions: Make Screen Readers read the number of replies and unread replies

Fixes CNVS-11719

Test plan:
* Using a screen reader, visit a discussion page, with and without replies,
  and with and without unread replies.
* Verify that the number of replies and number of unread replies are read,
  with context (I.E. text that says what the numbers refer to).

Change-Id: Iab57637c162fec04ca3fcb3c1681e42955ec384a
Reviewed-on: https://gerrit.instructure.com/31875
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>