Support queries by country label via separate geonames documents and parent-child queries

See #98

Deployed to staging: http://staging.api.lobid.org/oer

All fields in the geonames data are used by the filter, so different languages are supported:

http://staging.api.lobid.org/oer?q=*&location=germany
http://staging.api.lobid.org/oer?q=*&location=deutschland
http://staging.api.lobid.org/oer?q=*&location=ドイツ