In order to be able to split geojson into multiple leaflet layers, so that we can give links to data sources, we need to do a minor restructuring of the geojson parsing. The simplest way would be to force geojson to always be an array:

[
  { "type": "feature", ... },
  { "type": "externalData", ... },
]

should not be converted to

{
  { "type": "featureCollection", "features": [
    { "type": "feature", ... },
    { "type": "externalData", ... },
  ]
}

Instead, we should force all geojson to be an array of elements, and only allow the top level element to be an "externalData". This way, each element can be made into its own leaflet layer, and we could provide layer-by-layer contribution links, or links to wikidata query, etc.