Add notice feature from IRC

One good thing that I miss from the good ole IRC days is being able to /notice people and it's a message only they can see that can be sent in a general chat. It's a good way to have side conversations or ask things without having to move over and private message them. I know this would probably be a hard thing to implement with how discord is implemented, but maybe making it so notices are not saved in message history would be a good compromise.