Determine map server to use with Leaflet

[alisonrclarke]

Leaflet defaults to openstreetmap but its tile services are limited.

Check with Sim whether there's another tile server we already have access to or whether we need to set up our own (e.g. see https://django-wms.readthedocs.io/en/latest/views.html#tile-map-server-xyz )

[alisonrclarke]

Found a way to integrate Bing maps into Leaflet at Sim's suggestion.

[alisonrclarke]

Reopening as Bing maps are also rate-limited.