[feature] Allow to configure leaflet referrerPolicy

[nemesifier]

When using tiles which have an access token tied to a particular domain or URL pattern, the leaflet library is not sending the http referer and the tile requests return 403 Forbidden if the web server is configured with strict referer policy (eg: we use "same-site" in OpenWISP deployments by default).

In order to fix this we need to find a way to set https://leafletjs.com/reference.html#tilelayer-referrerpolicy at JS level. It's possible we may have to patch https://github.com/makinacorpus/django-leaflet to get this working.