Include new `coverage` field in `NwBibSpatialQuery`

We display `coverage` as `Raumsystematik`, so we should make it searchable too (`nwbibspatial` query param, `Raumsystematik` in UI).

See https://github.com/hbz/nwbib/issues/66

Deployed to staging:
http://test.lobid.org/resource?format=full&q=Kohle&nwbibspatial=Dortmund