Add label to announcement checkbox

Fixes: CNVS-12850

Added label to announcement checkbox to make it more
informative for screenreaders.

Test-Plan:
-Navigate to announcement tab within courses
-Inspect checkbox next to an announcement
-Will see the aria-label as same as announcement
  title.
-Enable voiceover
-Verify that voiceover prepends announcement title
  before saying checkbox

Change-Id: I486ce624c76382db817ebcd5e654838995d040e5
Reviewed-on: https://gerrit.instructure.com/34344
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
Product-Review: Jayce Higgins <jhiggins@instructure.com>