Allow selecting alternative basemaps

[BitScout]

Satellite imagery would be great.

Providers: https://leaflet-extras.github.io/leaflet-providers/preview/

• Esri seems to be limited to 2 million tiles and gets expensive after that.
• The French state's Géoportail has an unclear license and probably needs discussing with them if their map can be used.

[BitScout]

Contacted the Dutch Het Kadaster on Facebook. https://www.facebook.com/HetKadaster