* Add COURSE_ENROLLMENT_CHANGED: sent after the enrollment update
* Add COURSE_UNENROLLMENT_COMPLETED: sent after the user's unenrollment
* Add CERTIFICATE_CREATED after the user's certificate generation has
been completed
* Add CERTIFICATE_CHANGED: after the certification update has been
completed
* Add CERTIFICATE_REVOKED: after the certificate revocation has been
completed
* Add COHORT_MEMBERSHIP_CHANGED: when a cohort membership update ends