Allow users to allow or mute notifications per individual role

Roles are useful as both ways to distinguish users in a server and to collectively notify everyone with that role. However, in certain cases where they are used as both, I find myself wanting to mute notifications for a certain role, especially if I'm active in multiple servers. Since we're already able to put a blanket mute on all @role notifications, I think being able to fine-tune which roles get muted would be a useful evolution to that option.