One-to-one messages

[haadcode]

It should be possible to send a direct message to a user. Ie. 1-to-1 conversations.

1-to-1 conversations could be prefixed with '@' instead of '#' (channel). These could be expressed as orbit-db feed names with '@user1-to-@user2' where the order of usernames is deterministic (alphabetical).

There's a lot of UI work to make this happen.