Mail Activities: log on unlink

This module adds an automatic trace when activities are deleted, so nothing disappears silently from a record's history.

• Whenever an activity is unlinked (deleted) from a record, a message is posted on that record's chatter. The message shows the activity type and icon, the activity summary, and, if the activity was assigned to someone other than the person deleting it, the name of the original assignee.
• If the activity had a note, the message also includes the original note text under a "Original note:" heading.
• The message is posted under a new internal message subtype called "Activities unlink", so it can be filtered in the chatter.
• Deleting the parent record itself does not trigger these messages: the module skips the log when the deletion comes from the record's own removal, and also when an activity is completed through the normal done action.

It touches the activity model mail.activity and the activity mixin mail.activity.mixin, so the behaviour applies to every Odoo record that supports activities (sales orders, contacts, projects, etc.). There are no new menus or screens; everything happens in the existing chatter of the record.