feat: Make sure games are deleted in specific cases

[touficbatache]

• If the owner presses back after creating a game, and before anyone joining (done in feat: Delete game if owner leaves SecretHitlerSvelte#26)
• After a certain timeout if there are no players
• etc?

[touficbatache]

It's easy to detect presence in Firebase, especially using Functions & Realtime DB, which we both use : https://firebase.google.com/docs/firestore/solutions/presence

Simply, if we detect disconnection :

• if game status is waiting (has not yet started), check if every user is disconnected => delete it
• if game has started but someone is disconnected, show it in the app, maybe with a wifi off icon on the player's image (todo in Svelte)
• if game is public, has started, hasn't yet ended, and all are disconnected => remove it from watchable games