Use GNIS feature IDs in county JSON

[waldoj]

The JSON file providing geodata for all counties (counties.geojson) shouldn't just use census IDs (which it does now, because the data was extracted from census records), but also use GNIS IDs. That's because our place records (gnis_places.json) all use GNIS IDs, and it'd be good to be able to use the same IDs for all geodata.

That said, our GNIS features include census county numbers, so those can be used to ID counties within counties.geojson. Still, it's worth carrying GNIS IDs across files.