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.
I want to make sure to only include a) topics which the user can choose and b) layers which the user can choose from the catalog. Layers listing don't have any information regarding catalog.

Also, the sitemap will contain all catalogs and their layers. Theoretically, this will gives more weight to layers which are in multiple projects