discussions: fix graded discussion parsing

fixes CNVS-11738

test plan:
  * create a graded discussion
  * pin and unpin it on the index page
  * verify that it shows up in the appropriate section
  * regression test creating, editing, and viewing discussions

Change-Id: I425881f1d963cd18fcbe77823de4de0843333841
Reviewed-on: https://gerrit.instructure.com/31588
Reviewed-by: Jon Willesen <jonw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>