Clean up clients that refuse to quit properly

Clean up clients that refuse to quit properly

When a player is kicked or banned from a game, the client is expected to
disconnect in response. When they decide not to *or* the disconnect
packet is lost, Impostor just silently disposes of the connection.
This does not remove them from ClientManager, which is how I tracked
this bug down.

By sending them a proper disconnect message, clients should have
received *a* disconnection message before dropping their connection.

Future work should split this entire method in removing players from a
game and processing kick/bans because the latter feature is not relevant
for other callers of this method.