Close other socket connections when user joins

[goto-bus-stop]

When a user joins a server from different browser tabs or devices at the same time, any mutations they make will cause the other devices to show stale data.
Long term we should address this by sending websocket notifications when state changes on the server side, so clients can always stay up to date.

That is a lot of work, so in the mean time we could shut the other connections down so the user has to reload on other devices before doing anything else.