fix updating state of graded group discussions

test plan:
* create a graded group discussion
* as a student reploy to the discussion (thus ensuring
 that it cannot be unpublished)
* closing it for comments or pinning it on the
 group discussions page should work

fixes #CNVS-13904

Change-Id: Ia02222b578be15b22dd1cc267cf26771032be266
Reviewed-on: https://gerrit.instructure.com/41786
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>