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.
 a) topics which the user can choose
You will just need to filter out api-free, api-notfree  and swissmaponline right?
b) layers which the user can choose from the catalog
Except for `ech`, if a layer is in a topic it is also in the catalog. @AFoletti could you confirm?