implement theoretically sound collaborative editing

[travis]

The current implementation uses auto-save and patch semantics to provide a rough collaborative editing experience. It will definitely be frustrating for more than 2-3 people to edit a document together with the current implementation.

I worked up a basic proof-of-concept using Automerge, but the underlying datastructure grows very quickly in a way that makes in infeasible as the at-rest storage format for a document:

5ba932d

We may be able to use this as a temporary datastructure for collaborative editing sessions, or there may be better approaches.