Treatment of GeometryCollections in map UI

[rsimon]

Places which have a GeometryCollection as geometry currently don't work properly, either in the georesolution nor the map UI. As a first measure, make sure that point-only GeometryCollections are reduced to points (#554) and then enhance the handling of GeoJSON overlays in the map UI (which is what GeometryCollections get converted to), so we can handle cases where geometry counts as more than a single Leaflet marker.