Add ability to set a default encryption method for new chats
Currently, one has to explicitly enable OMEMO/GPG encryption for new chats. This can easily be a footgun since gajim will happily send unencrypted messages without an explicit warning.
It would be great if one could set a different default than 'plain text'. Conversations has this, Dino has an issue for it.
When !878 (merged) is merged I would also recommend to make OMEMO the new default.