Entitlement detail pages in django admin normally show a dropdown
button for the enrollment_course_run field. But on stage, that can
cause a timeout because the enrollment database is so large.
So instead, just show it as a raw id field.

The Entitlement API will allow for GET, POST, DELETE(Revoke), and
variations of these methods of managing and retrieving data about
Learner Entitlements.
[LEARNER-2661]

[LEARNER-1733]