DA - course dashboard recent activity

fixes CNVS-14101

test plan:
 * setup DA
 - go to a course home page
 * as a student
 - 'recent activity' should not show notifications about
   assignments/discussions that are not visible to the student
 * as a teacher
 - 'recent activity' should show notifications for all
   assignments/discussions

 - make sure it works normally with DA off

Change-Id: I7d4cad8fd6e6cc5ed4dc48ed139182bcc59652eb
Reviewed-on: https://gerrit.instructure.com/40722
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>