Improves filter checkbox labelling

fixes CNVS-13880

Added aria-labels to the checkboxes so their effect was clearer to
screenreaders.

Test Plan:
* Using a screenreader go the discussions page and navigate to the
  "Unread" and "Assignments" filter checkboxes
* Using a screenreader go to the announcements page and navigate to the
  "Unread" filter checkbox
* Using a screenreader go to the home page, select an announcement and navigate to the
  "Unread" filter checkbox
* Ensure that the checkboxes are descriptive of the function the
  checkbox performs from the screenreaders perspective

Change-Id: Ie27d2dd3905e55c64b59c05b22d56926bf6bb2b6
Reviewed-on: https://gerrit.instructure.com/45286
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>