Allow override for Nominatim base URL (v5)

[bplubell]

Nominatim can be self-hosted or one might want to use a proxy, for example to implement rate limiting. To support such use cases, I made the Nominatim base URL a setting. The default value is the same origin as was hard-coded in the GeoService requests, so this feature is backwards compatible.

Notably, being able to use a rate-limiting proxy helps users ensure they adhere to Nominatim's usage policy.