Set idle players as AFK, also allow players to manually declare themselves AFK

[robhawkes]

Sometimes players want to stay in the game but don't want to keep getting shot at. Introducing a system where a players can set themselves as AFK so they can't die would solve this. This system could also be used to automatically set inactive players as AFK so they don't clutter the game.

AFK players could be made a different colour or slightly transparent, they could also have "AFK" next to their name.

Restrictions should be put in place so players can't constantly set themselves as AFK to avoid being shot.