[MB-1077]
- Stop removing the verify_uuid when revoking course certificates. In v2 of course certificates this will allow us to retain the same URL to the certificate. If a learner's certificate is revoked (from being invalidated or no longer passing the course) the UUID will remain intact and the URL will not change.
- Look for an existing cert for the user in a course-run during generation. If one exists, use the UUID from the existing certificate when updating the record.
- Stop generating a `key` for v2 web-certificates. This is not needed for web-certs (this is used in PDF cert generation).