[e-Newsletter] Allow for tracking of UNsubscriptions

Ref: https://premium.wpmudev.org/forums/topic/e-newsletter-improvements-to-subscribe-and-unsubscribe-processes

There’s no report to show subscribe/unsubscribe activity. Please add this, and/or provide a hook so that we can take action on these events. It’s important to know when we lose someone, and what they were reading when they made the decision.

For a hook I think we would just need the user ID, email address (if they’re not a member), and the Newsletter ID which led to the UNsubscribe request. From there we can use a custom function to get a timestamp, log other similar events, establish patterns of subscriptions and cancellations, etc.

I will look at the code as time permits, but it would help if a developer could point to existing hook, and suggest the ideal place to add a do_action for a custom hook for this, until a formal enhancement is added (if ever).