calendar: handle concluded checked courses

fixes CNVS-12384

test plan:
  * create two courses
  * open calendar 2 and show events from both
  * conclude one of your courses
  * open calendar 2
  * verify that events from the remaining course are displayed

Change-Id: Ia17b0e0391aed343af2c45dc62a4bc9284830259
Reviewed-on: https://gerrit.instructure.com/33262
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>