R1:0dee34b3fafd

Make Facts more modern, DRY, and dimensional

Summary:
Ref T13083. Facts has a fair amount of weird hardcoding and duplication of responsibilities. Reduce this somewhat: no more hard-coded fact aggregates, no more database-driven list of available facts, etc. Generally, derive all objective truth from FactEngines. This is more similar to how most other modern applications work.

For clarity, hopefully: rename "FactSpec" to "Fact". Rename "RawFact" to "Datapoint".

Split the fairly optimistic "RawFact"…