Implement unregistered recipients

This one has a lot going on:

• Recipient can now exist and be in unregistered state
• Don't fetch profiles for unregistered recipients
• Expose is_registered in QML (disallow direct send, show in progile)
• Don't try to send group messages to unregistered group members
• Don't try to fetch profiles of unregistered recipients

The starting point of all this was when I noticed that I get a 404 response whenever I send a message to #whisperfishers group. It turned out to be a member of the group who is no longer registered. Then the ball started rolling... 😄

Requires https://github.com/whisperfish/libsignal-service-rs/pull/215

• Try on #whisperfishers

Food for thought:

• What if same UUID gets re-registered?
• What if UUID is unregistered, but a message arrives from it?
• Should we anonymize the unregistered profile?

Fixes #531 (closed)