Use bespoke SignalR mapping code (#23)

* Use bespoke SignalR mapping code

OpenAPI doesn't do the best job of representing how messagepack actually packs the objects we receive from SignalR. Rather than generating mappings, I've just written some plain-old mapping code that goes from messagepack to the definitions that we use through the rest of the app. `docs/fms/Hub Documentation.md` is the reference for the precise layout of the structures here.

* Really prettier