Fixes#3205. Added caching of nominatim URL query parameters in sources global variable (as parameters parameter) in GeocoderController#search for both direct and reverse geocoding. In app/views/geocoder/search.html.erb added displaying cached URL as forwarding link when clicked on "OpenStreetMap Nominatim" label. Updated GeocoderControllerTest to check only name (latlng, osm_nominatim, osm_nominatim_reverse) parameter of new sources variable.
The geocoder backend calls are a private implementation detail
and using POST enables request forgery checks and makes scraping
of nominatim via the backend calls harder.
Administrative boundaries at municipality level are generally
better described by the place node (as town, village etc.).
Nominatim exports the place type in extratags if it was able
to merge place nodes with admin boundaries. Use this preferably
to create the descriptive term.
