fix assignment edit form submission handling

adds type=button so that submit event doesn't trigger a click on a
hidden button

fixes CNVS-7542

test plan
=========
using Firefox (but should work in other browsers as well) do the 
following:
- go to assignment edit page and focus the points field
- press enter
- verify that the assignment saves
- verify that a new due date field is not added

Change-Id: Icaa0707cb75fedd44ee2c4e267695ffc6ed0755b
Reviewed-on: https://gerrit.instructure.com/30042
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Matthew Irish <mirish@instructure.com>