allow pressing enter to add module items

fixes CNVS-12660

Test Plan
As a teacher
Given I have a module
And I've clicked the add module item + button to add a module item
When I fill out any of the forms
And I press enter
It should add a module item
And it should add the module item once.

As a teacher
Given I have a module
And I've clicked the add module item + button to add a module item
When I select an existing module item with my mouse
And I press enter
It should add a module item
And it should add the module item once.

Change-Id: Iebc25a1906519f16af36686e0d20ba3094ed6ba3
Reviewed-on: https://gerrit.instructure.com/34515
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>