add the ability to use a subpath instead of subdomain

[JokerGermany]

I don't know if something has to be changed for traefik, i am running this with this vars.yml

paperless_hostname: examle.com
paperless_subpath: /paperless

and this nginx config:

	location /paperless {
		proxy_pass http://127.0.0.1:8731;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection "upgrade";

        proxy_redirect off;
        proxy_set_header Host $host;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Host $server_name;
        add_header Referrer-Policy "strict-origin-when-cross-origin";
	}

[JokerGermany]

This should also work:

paperless_subpath: /paperless
paperless_environment_variables_extension: |
  PAPERLESS_STATIC_URL={{ paperless_subpath}}/static/
  PAPERLESS_FORCE_SCRIPT_NAME={{ paperless_subpath }}

I let it open to let others decide if it should be an easy accessible variable.