FR: Stats: Track Unsubscribes per Campaign

Context:

• In an effort to part-out some of the ideas here: Issue 2692 and make things easier to track I'm creating this Feature Request

Problem:

• Listmonk doesn't track unsubscribes per campaign, only views.
• A future campaign stats dashboard will require more data to be tracked.

Possible Approach (Unsubscribe):

• As there is a campaign_views table, there could be a campaign_unsubscribes table
• With id, list_id,campaign_id,subscriber_id,date_unsubscribed
• Records are added on every unsubscribe, per list, with the campaign_id that caused them to unsubscribe.

Possible Approach (Subscribe):

• If a user of id ever re-subscribes, the matching subscriber id record could be deleted

This would provide per list and campaign unsubscribe detail that could be added to a future stats dashboard.