ssbc/ssb-peer-invites

Peer invites is a new invite system for ssb that allows users to create invites without needing a pub server. It works by having the host generate an invite message with optional private and reveal sections, then providing the guest with an invite code containing the seed, invite message ID, and pub addresses. The guest can then open the invite to view any private message from the host and see what will be revealed upon accepting. If they accept, they publish an accept message which is confirmed by a pub, allowing the guest to be replicated by the host's friends. The system provides more accountability than the previous followbot system by making it clear who invited who.