Sitemap service for map.geo.admin.ch

https://github.com/geoadmin/mf-chsdi3/issues/849

This will replace current static sitemaps in map.geo.admin.ch. Note that another PR in ga3 is needed to use this service.

But it doesn't hurt to review/merge this now.
If you really want to use the catalog information, use the catalog table.
You won't have to handle the recursive mess.
See: https://github.com/geoadmin/mf-chsdi3/blob/master/chsdi/models/bod.py#L313