Backup system

[fenhl]

Clients should keep backups of all rooms to allow players to restore closed rooms or move to a different host. Don't keep backups on the dedicated server.

[fenhl]

This is especially useful now that rooms are automatically deleted for inactivity. A UI element to create a new room from a backup of an existing one might be the best way to expose this feature.

[fenhl]

Don't keep backups on the dedicated server.

I'm not sure why this was a concern anymore. Keeping backups on the server by simply removing rooms from the list after a while but only actually deleting them much later might be easier to implement. Unsure how to control access to these backups though.