Fix existing sandbox list in Start a New Course form

The User model was recently refactored to use scopes instead of associations with joins (see [related commit](https://github.com/instructure/canvas-lms/commit/57f6ef2d7d9817f19b080ec6a845b5fb2800c8b0#diff-4676c008b11a5480d73d4a6de01e45b9L82)). 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.