Implement server notices (usermode +s)

This implements the server notice mechanism. It adds umode +s which acts as a listmode and keeps track of what users are subscribed to what events. It supports using multiple mode parameters using commas and can list what events the user is subscribed to. I had the idea of splitting the oper permission into different permissions for each event with a "servernotice-all" wildcard permission, but I decided that might be overdoing it a bit (though I am still willing to implement it if you would like me to :P).