dashboard: hide stream items from concluded courses

fixes CNVS-1485

test plan:
  * verify that stream items show up on the dashboard
  * create a term with an end date in the past
  * assign a course to that term
  * verify that items from that course are no longer on the dashboard

Change-Id: I9851997c9672f73335f54bdba78bdb71ee24d029
Reviewed-on: https://gerrit.instructure.com/33264
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>