Use Observables for detecting new builds

The old system looked a bit like Observables if you squinted, but didn't allow for the kind of simply testable transformations that Observables do.

When I was looking at adding other CI systems, I found this really difficult to keep all in my head.

I haven't yet tried to go the whole hog of getting all data from Teamcity, but if this is merged, that probably makes sense in the longer term.