Add expiration dates to Coupon Codes

@chrisndodge @afzaledx Kindly review the changes for WL-135

Thanks
Hi, unfortunately, you aren't using the correct term in this filename and various comments/names. The right name to use is "Paid Course Registrations" use-cases and not Paid Certifications (which I think other people will interpret as Verified Certificates).

Can you change the various names in the file as well as the filename as well?