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 have a look if I can use the catalog table directly tomorrow. (I don't have db access now).

a) yes, but why use a filter if we have a service already that does exactly what we want. I don't think that's easily maintenable.
b) I don't understand the exception you mentioned. Per topic, I only want the layers in the sitemap which appear in the topics catalog.