prevent notifications when a course is soft concluded

fixes CNVS-4348

test plan:

- have a course that has an end date in the past

- create an announcment, it should not send a notification
- create an assignment, it should not send a notification
- create a discussion topic, it should not send a notification

Change-Id: Ic371d651ae00abe80c17e569f74e94e49c017708
Reviewed-on: https://gerrit.instructure.com/36229
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>