Add `autosubscribed_users` feature
So you know how we can have autofollowed_users and autofollowing_users? Imagine if we had autosubscribed_users. When a new user signs up, they would be automatically subscribed to receive notifications from the chosen accounts.
This is basically a way of setting up makeshift admin announcements except it's even better than Mastodon's version allowing for maximum flexibility. You could create an account representing your instance and set it to be autosubscribed, and you'll never have to worry about users missing important messages about downtime etc.