Unsubscribe links and page

When sending a notification, include a link that contains information about what triggered the given notification and which opens an unsubscribe page, where the user can decide what to unsubscribe from.

For example they can unsubscribe from the event that triggered the notification, or the event's parent, or the event's parent's parent and so on

No files, tests, or entry points are named. Start by locating notification generation and the existing event hierarchy, then trace how web pages are structured; done should include a notification link carrying its trigger context and an unsubscribe page where users can choose the event or an ancestor event.

Written by the indexing model from the issue text.