Fix existing sandbox list in Start a New Course form

The User model was recently refactored to use scopes instead of
associations with joins. That change eliminated the User#course_ids
method, and caused the Start a New Course form to not display any
existing sandbox courses.

The fix is to use the new User#courses method to get the course IDs.

Related commit: 57f6ef2d7d9817f19b080ec6a845b5fb2800c8b0
Merge pull request #192 from va7map/fix-course_form-sandbox

Fix existing sandbox list in Start a New Course form