Support different domain for user addresses

[raucao]

Technically, anyone can server Webfinger records for a different domain and point them to their Armadietto instance. However, it would be nice if admins could configure it in a way that the UI also tells the user to use e.g. the main domain name instead of Armadietto's subdomain.

Example after e.g. installing Armadietto on Cloudron:

I guess it means adding a new config that would be used by the UI to show a different user address. And perhaps showing the domain in the top left corner isn't the best idea. Maybe just having "Armadietto" there makes more sense (even without any other changes).

I guess whatever the built-in solution is for the UI, it would have to include some tips for admins for how to easily serve the records. One option, in case they don't have any other Webfinger records for their main domain is to just forward requests from the Web server serving the main domain for example (in this case nginx):

  location /.well-known/webfinger {
    proxy_ssl_server_name on;
    proxy_set_header X-Forwarded-Host $host;
    proxy_pass my.rs.subdomain.example.com;
  }