Don't allow unknown submission_types in assignment[submission_types]

Fixes CNVS-14578

Test plan:
- Create (or update) an assignment, passing in a submission_type that isn't
   known to Canvas, i.e. assignment[submission_types][] = 'really_awesome_submission_type'
- Verify 400 error is returned, along with error message

Change-Id: I2d0fdf785f8cd9fdee014258878197c4af9ca3d7
Reviewed-on: https://gerrit.instructure.com/38730
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>