This commit adds city data to imported records from Google Analytics. The
current implementation sets city to 0 because GA does not use the GeoNames
database.
Google Analytics Reporting API uses [Geographical IDs](https://developers.google.com/analytics/devguides/collection/protocol/v1/geoid)
to identify cities and countries. Plausible uses
[GeoNames](https://geonames.org/) and I couldn't find databases corelating the
two.
Fortunately, GA also returns the city name and this commit uses the city name
and the country ISO code to find the Geoname ID. To avoid making expensive ETS
searches, I created another ETS table in the Location library that uses
{country, city} as a key.
Related PR: https://github.com/plausible/location/pull/3
