Auto-start a public lobby shortly after the host leaves

A host can fill a public lobby and then leave, stranding everyone who was waiting. It happening overtime degrades player trust and can make them stop bothering with public private lobbies all together.

Host migration was intentionally disabled, and bringing it back has its own problem: the new host could change the settings people actually joined for.

Suggested instead — no host migration, just **auto-start the game ~30s after the host leaves**. The settings stay exactly as they were when everyone joined, and nobody is left sitting in a hostless lobby.

Optionally, a cooldown on creating a new lobby for a host who repeatedly abandons one, since a single troll opening and dropping lobbies all day is enough to make people stop joining them.