Map: Add reusable admin source

## Notes
 
- needs some way to auto-update the data
- laender, kreise, gemeinden
- choropleth demo
- should be versioned so old charts don't break
- https://daten.gdz.bkg.bund.de/produkte/vg/vg250_ebenen_0101/
- https://docs.mapbox.com/data/boundaries/reference/mapbox-boundaries-v4/
- https://demos.mapbox.com/boundaries-explorer/?country=DE&layer=adm1&worldview=US#6.23/51.156/10.928

cc @sjockers 