make message disappearing clearer

[yippyskippy]

currently messages auto delete after 4 days. this is not communicated to a user. since this is privacy messenger, maybe keeping it disappearing by default is better? at least the app should

• on a empty chat screen show Messages delete after 4 days at the top

[mixmasala]

messages are expunged on a rolling window, so the notification should be on each message

[mixmasala]

i like the idea of being able to configure the message expiration duration per conversation but we do not have a mechanism for clients to indicate to each other that this has changed. there isn't really a way to guarrantee this, but it might be worth implementing regardless.