[Accessibility] Join and leave group accessible notification

Fixes: CNVS-12811

Needed a notification for joining and leaving groups
that is accessible. Using flashMessage method this
is easily done and keeps these notifications standard
across the application.

Test-Plan:
-Activate new groups feature flag and create a group
  --Make sure the group can be joined manually by
     users
-Login as a user and join and leave the group
-Make sure that the notification is accessible

Change-Id: I5921c362dc4830565a95b80e193c2248efc232d9
Reviewed-on: https://gerrit.instructure.com/36210
Reviewed-by: Braden Anderson <braden@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Jayce Higgins <jhiggins@instructure.com>