Update management command for user deferrals to include courses with closed enrollments

[cachob]

As a learner I'd like to be able to transfer to a course run even after the enrollment period ends. This can happen as a special request if I had signed up for a future run, but would like to join the most current run of the same course to better suit my schedule.

As a site admin I'd like to be able to run the defer_enrollment management command to accomplish this, without having to do any extra steps.