Adds check for existing upgradeable enrollment on user entitlement
creation, if single upgradeable enrollment is present, upgrades enrollment
to entitlement mode and links the entitlement to the upgraded enrollment
LEARNER-3579
[LEARNER-2668]
These changes will support a user intiated Revoke and Refund of
an Entitlement. This includes unenrolling the User from
any related Course Runs they are currently enrolled in.
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]