Partial replication

This version is a major rewrite of the core module that underpins the ssb browser demo. The database backend has been rewritten to use jitdb and a custom single index optimized for performance. This allows one to get much closer to a normal ssb client, where you download the full contact graph including blocked contacts and about messages for profiles.

This release adds backlinks on messages and showing forks of threads. Furthermore it properly handles blocked contacts.

Sadly this also means that peer invites had to go, for now at least. I have also removed the collaboration feature, mostly because it was a fun experiment and didn't want the client to become a big kitchen sink of features.