Feature request : Implement confirmation and feedback/reason to unsubscribe when doing so

[MagicFab]

When unsubscribing Keila will remove the email immediately.
In some cases it's desirable to have a check box or additional step to confirm unsubscribing, along with the reason for doing so, as a short list of reasons plus free form text.
This reason for unsubscribing would be useful to keep as a metric too.