"Subscribe" to a user for notification of every status

Requesting BE functionality to "Subscribe" to a user for notifications of every status they post. No new timelines, this is simply notifications.

• Pleroma-specific API endpoints for subscribing and unsubscribing to a user
• Trigger notifications for each status to all subscribers
• Filter out duplicates in case users that subscribe are also mentioned in the status

This would allow me to get notifications for every status posted by @lambadalambda regardless of whether or not he mentions me 😍