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.
You're looking for layers only at the first and second level of the tree. For instance I don't find any layers in 'lufbilder'.

I cannot find `ch.bafu.oekomorphologie-f_bauwerke` also, that should be in bafu topic.