fixes assignment creation with quick add dialog

fixes CNVS-15217

test plan
- create an ungraded assignment using the quick add dialog
- do it again using the 'quiz' or 'discussion' type
- do it again and click 'more options'
- save the assignment with a single online submission option
- verify that it saves/works for every assignment you created
- party party celebrate victory wooo :thumbsup:

Change-Id: Ib4c924801d3f3ed228971151d88c97b7bc572fb1
Reviewed-on: https://gerrit.instructure.com/40416
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>