add screenreader content to read/unread pills
add screenreader content to read/unread pills

fixes COMMS-1022

Test Plan:
- create a discussion or announcement with read and unread
  replies on it
- go to the announcement or discussions index page with the
  SSA or SSD flag turned on
- with a SR navigate to the pill elements
- notice they read what they are accordingly
- navigate to the rest of the elements on the row
  and notice they still also read correctly

Change-Id: I3337719da0934268d96634b033abbdc4b06473d2
Reviewed-on: https://gerrit.instructure.com/145670
Tested-by: Jenkins
Reviewed-by: Venk Natarajan <f9eab59b292314637a3c917fea4742741e36a19b@instructure.com>
QA-Review: Aaron Kc Hsu <089edc94045de3d9f5f372fb17706d21afcc259f@instructure.com>
Product-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>

sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms