Add a confirmation email to the Subscription block when a user cancels their subscription

### What

The user offers dance classes using the subscription block and is requesting the option that their subscribers receive a confirmation email when they cancel the subscription.
4794240-zen

### Why

It can be confusing to subscribers whether they are actually unsubscribed or not.

### How

A confirmation email is being sent when a user subscribes so why not have an email being sent automatically when the user unsubscribes.

Start by locating the Subscription block's existing confirmation-email flow for new subscriptions and the handling used when a subscriber cancels. Confirm the intended cancellation state, then verify that cancellation sends a confirmation email without changing the existing subscription notification.

Written by the indexing model from the issue text.