resolves group discussion entries not appearing in gradebook and speedgrader

fixes CNVS-17632

test plan:
- create a graded group discussion
- add an entry to the group discussion as a student that is part of that group
- the gradebook should show a submission for that student
- the student's entry should also appear in speedgrader

- go back through test plan from gerrit.instructure.com/#/c/45461
- ensure that test plan still passes

Change-Id: Ib47a9c1299c8fa4083be19a61337dfc26060c36e
Reviewed-on: https://gerrit.instructure.com/46315
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>