Add capability to use a WMS server as basemap

[jemacchi]

I would like to offer next modifications which allows to extent the basemap capabilities in the nominatim-ui tool, to use a WMS tile based service. I have applied that in context of this project: https://github.com/geotekne-argentina/osm-nominatim-geoserver-postgis

(hope you find it useful -it was for me- since I expected to show that all the data can be consumed from local resources).