Accept updated ungraded assignments

In our array of allowed submission types, we were missing the option for
"not_graded", which was causing the API call to fail.

Fixes CNVS-14935

Test Plan:
    - as a teacher
    - create a new assignment using the 'more options' link
    - set grading type to 'Not graded'
    - verify that the assignment saves

Change-Id: I382b669f6064d925d70a6533c4e3b2b938dec76d
Reviewed-on: https://gerrit.instructure.com/40033
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>