feat: keep users who have left the room in the scoreboard

[marwanhawari]

Right now, when a user leaves the room, they are no longer displayed on the scoreboard. This means I can't look at the solutions of people who have solved the problems but left the room.

I want to change this so that I still see the users on the scoreboard even after they've left. I already retain the users submission data for the room even when the leave (that way if they rejoin the room their submission data is still there). The backend just needs to make sure to include the people who have already left and have a way to indicate that they have left.

I also want a visual way to distinguish between people on the scoreboard who are still in the room and those who have left. Maybe some change in font color or opacity?