Improve guessing of labels for Nominatim results

This adapts to two changes in place handling by Nominatim:

* Place links that are computed by Nominatim are now found in
  extratags['linked_places']. Keep the check for extratags['place']
  as this may contain an explicitly mapped place label.
* Use address rank for guessing the admin level. This gets normalised
  by Nominatim with respect to country-specific use of admin_level.

Also adds additional labels for admin levels, so that we now have
complete coverage for all levels that are in use in OSM.