Add a command to transfer students from one course to another.

Move only active students and also update cert item for verified students.
CertificateItems should be updated for all course students.

Not just students that are currently active.  Students may have enrolled with verified certs, paid, gotten
a refund, and un-enrolled from the original course.  This function should now capture that.
Save state before unenrolling student from old course.
Merge pull request #2041 from edx/feanil/transfer_students

Add a command to transfer students from one course to another.