Implement private lobby codes for private games

Even though private lobbies do exist, it seems like no one really knows how to directly invite their friend to a game over Steam, so instead they default to inflating the Multiplayer's public lobbies with public games (anyone can join), and immediately kick out anyone who isn't their friend. It's annoying to handle when you're looking to have a good time with random players and having to check 20/32 available servers before you get one that doesn't kick you.

To fix this, we should have a custom lobby code for each private lobby-- something like a six-digit string of characters (example: XH709J)-- easily visible near the top of the lobby so that friends can directly invite each other without inflating the public games.