* Implement permissions specs for mentions.
- public boards: admin, editor, commenter can mention team members and auto-add them to board; guests can mention board members
- private boards: admin, editor, commenter, guest can mention board members
- viewers cannot mention
* Don't notify non-board members of card changes
- include permissions service in notifications backends
- use permissions service to ensure @mentions are to users on team
- use permissions service to ensure subscribers are members of board
Backend support for subscribing/unsubscribing to blocks, typically cards and boards. Notifies subscribers when changes are made to cards they are subscribed to.
