Handle NYC counties

Alone among US cities, the NYC "locality" contains more than one "county," since each borough is a separate county. Right now, `locality -> county` is represented as a single one-to-one FK.

To model this, we'd need to use some kind of separate `<county, locality>` mapping table? But it's weird, since in all other cases county is a parent of locality, but for NYC, the locality is the parent.