Bug fix for pressing enter key on add course form

Before this fix, if your pressed or held down the enter key
when creating a course on the Add Course dialog, duplicate
courses would be created.  This prevents that from happening.

fixes CNVS-13480
fixes CNVS-14154

test plan:
- Go to one of your accounts
- Click the "Add a New Course" button
- Put in a Course Name and Reference Code
- Press Enter
- The form should only submit once and only one course
  should be created.
- Go to /courses
- Click Start a New Course
- Enter a Course Name and press Enter
- The form should submit and take you to that course's page
- Go back to /courses
- There shouldn't be duplicates of the course you just created

Change-Id: I32980445e2e6748188f84232cb8c0740c660c8a0
Reviewed-on: https://gerrit.instructure.com/41825
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Dan Minkevitch <dminkevitch@instructure.com>