sets default to online submission for new assignments

fixes CNVS-10387

test plan
- create a new assignment
 * from assignment index with draft state disabled
 * from assignment index with draft state enabled
- verify that 'Online submission' appears on the form instead of 'No Submission'
- edit an existing assignment
- verify that correct submission method is listed

Change-Id: I89c678d285bb36192779946c6550edb8e1f1539e
Reviewed-on: https://gerrit.instructure.com/32294
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Liz Abinante <labinante@instructure.com>