Unenrollment popup + email notification for courses/memberships

**Is your feature request related to a problem? Please describe.**
When a student is unenrolled from a course or membership (ie. due to a missed payment), there is no notification. While the dunning process begins, admins and/or students may want to be notified when their access is revoked.

**Describe the solution you'd like**
Add a new "unenrollment" popup and email notification for courses/memberships, disabled by default.