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'll use the catalog table, but will filter by topics found from the topics service. This assures only topics available will be included.