Add counties/regions to filter map

[johanvonboer]

Would be cool if you could also get the county polygons on the filter map.

[johanvonboer]

Related: humlab-sead/sead_query_api#112

[johanvonboer]

I started implementing this, but there are issues:

1. When selecting a country, should that count as a "Country" filter selection? And if so, how do we handle selecting countries which that filter does not contain, e.g. Somalia?
2. The other option is to select the country as a polygon, but the problem with this is that many countries are not just one polygon, e.g. Denmark, so we would need server side support for selecting multiple polygons.
   2b. Another issue with this is that the country polygons I am using are fairly low resolution to save on space, which in some edge cases could result in a case where a site in a country could end up outside the country polygon.