DA - fix course home page

fixes CNVS-13626

test plan:
 * with the Differentiated Assignments feature on
 * as a teacher
 - navigate to a course home page
 > it should load without an error
 - create an assignment with an online submission
 * as a student
 - submit something for that assignment
 * as a teacher again
 - navigate to the course home page
 > it should load without an error

Change-Id: I912f3898b518fff693c5252879e78135ef4d62a7
Reviewed-on: https://gerrit.instructure.com/36426
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>