Self-destruct timer for messages

During testing session #788 (closed) and during the first Briar presentation at Cryptorave, users asked if we support a self-destruct timer for messages like Signal introduced recently and like it is also supported by Telegram.

The user seemed to see less need for the feature after it was explained that this feature does not help against adversaries who also receive that message, because they can always retain it despite the self-destruct timer. Most users might not know that and get a false sense of security from such a feature. However, testers still found it nice to be able to let messages delete automatically in case they are forced to enter their password or somebody gets hold of their phone unlocked.