searchable unsubscribe and opt-out activities

Clients want to be able to ensure that their base is not leaving the fold, so they want better tracking of unsubscribe and opt-out events.

Capabilities to provide:

• Configurable list admin notification on list activities, such as member list join, unsubscribe or opt-out.

• Searching on members' subscription status (subscribed, unsubscribed, opt-out). Clarification: while subscribed members appear in a group; it would be useful to be able to find all users that have unsubscribed from a group, or all members that belong to a group and have chosen to opt-out.