This introduces state in the ScoreboardManager for keeping track of the scoreboards that players have before the arena session starts, if any. Upon leaving, the scoreboards are restored.
Ideally, this change should be re-implemented as a Step of the join/leave process, but that would require opening up Arena or ScoreboardManager in ways that would probably be best handled in a rewrite of the scoreboards, so we'll leave it in ScoreboardManager for now.
Fixes#366
Andreas Troelsen
9809e9ebc9