Log or notify when a subscriber unsubscribes from a site

### Impacted plugin

Jetpack

### What

Send a notification, or write a log entry, when a subscriber leaves a user's site - similar to how (if memory serves) we currently do for new subscribers.

### How

Could either be a notification by email, or a note in the activity log. Ideally, it should be both and we should move new subscriber notifications to the same place.

Start by reviewing how Jetpack currently handles notifications for new subscribers, then compare the available email and activity-log approaches described in the issue. Clarify whether unsubscribe events should use one or both channels, and whether new-subscriber notifications should move there as part of the same change. Done means the chosen unsubscribe notification behavior and any related new-subscriber change are implemented.

Written by the indexing model from the issue text.