Enable end-to-end encrypted chats

[Peppe]

Enable having chats between users which are end-to-end encrypted. The server should not be able to read the message.

There are two alternative/complementing routes that can be taken here.

1. Like now the server accepts all messages and relies them, but the message is encrypted, and it is impossible for the app server to decrypt it
2. Form peer-to-peer connections between users and send messages directly, without the server every getting the message. This has obvious downsides that both users must be online at the same time and you can't ask the server for a log of previous messages. They would be stored in the client and lost if the client's state is wiped.