Update mapper to use countryless locales

Previously users could only send locales with a country (e.g. `en-US`); this lets users send countryless locales (`en`). It also remaps `country` from being a custom property (which looks to have been a mistake) and instead sends them as `country`, which is what Amplitude expects.

This also means we no longer send `country` as a custom property to Amplitude.