Remove Rack::Geo

This removes the Rack::Geo and Geogov double-act and replaces the few calls it made with a direct call to Mapit using a new API adapter added to gds-api-adapters.

This means we can remove a lot of cruft and simplify a lot of code, and I've also tidied up  the conditionals in the places template.

This also includes better end-to-end tests for places, and the associated work in gds-api-adapters makes available a handful of new test helpers for stubbing mapit and location requests.