Switch to using an external Ntfy role

The newly extracted role also has native Traefik support,
so we no longer need to rely on `matrix-nginx-proxy` for
reverse-proxying to Ntfy.

The new role uses port `80` inside the container (not `8080`, like
before), because that's the default assumption of the officially
published container image. Using a custom port (like `8080`), means the
default healthcheck command (which hardcodes port `80`) doesn't work.
Instead of fiddling to override the healthcheck command, we've decided
to stick to the default port instead. This only affects the
inside-the-container port, not any external ports.

The new role also supports adding the network ranges of the container's
multiple additional networks as "exempt hosts". Previously, only one
network's address range was added to "exempt hosts".

roles/custom/matrix-nginx-proxy/templates/nginx/conf.d/matrix-ntfy.conf.j2

@@ -21,7 +21,7 @@
 		{% if matrix_nginx_proxy_enabled %}
 			{# Use the embedded DNS resolver in Docker containers to discover the service #}
 			resolver {{ matrix_nginx_proxy_http_level_resolver }} valid=5s;
-			set $backend "matrix-ntfy:8080";
+			set $backend "matrix-ntfy:80";
 			proxy_pass http://$backend;
 		{% else %}
 			{# Generic configuration for use outside of our container setup #}