Spatial queries + filter improvements

 * add "sort by distance" to list filter
 * add `nearest(lon, lat)` scope to GeoData (used to sort by distance)
 * add `tile(x, y, zoom)` scope to GeoData (to be used by map component)
 * remove the use of "GEOMETRYCOLLECTION" because it is incompatible with ST_Intersects function
 * add list filter specs