used courses system + upgrade second-hand purchase system

[garoot]

1

create the functions for when a student purchase a used course

• createReceipt() which should already exist in user.controller.js
• setPrevEnrollment() in enrollment.controller.js to keep track of second-hand purchase timeline

[garoot]

2

create the functions for when a student upgrades a used course

• createUpgradedEnrollment() in user.controller.js which references the deactivated enrollment as prevEnrollment
• deactivateEnrollment() in enrollment.controller.js to turn off the first used-purchase enrollment and activate the new one with the upgrade
• createReceipt() which should already exist in user.controller.js and link to new upgradedEnrollment

[garoot]

done in receipt.controller.js