Fix nginx "add_header" directive inheritance issue

https://www.f5.com/company/blog/nginx/avoiding-top-10-nginx-configuration-mistakes#directive-inheritance

E.g. the Strict-Transport-Security header is "overrode" in multiple places. For HTTP/3 (#606) we need the Alt-Svc header, so this should preferable be fixed before that.

I think moving the "global" headers into a "headers" snippet file, is the easiest solution and then include it every time a header is added in e.g. a location block.