Update CORS for ma1sd

Even with the v2 updates listed in #503 and partially addressed in #614, this is still needed to enable identity services to function with Element Desktop/Web. Testing on multiple clients with a clean config has confirmed this, at least for my installation.

roles/matrix-nginx-proxy/templates/nginx/conf.d/matrix-synapse.conf.j2

@@ -64,6 +64,7 @@
 
 		proxy_set_header Host $host;
 		proxy_set_header X-Forwarded-For $remote_addr;
+		add_header Access-Control-Allow-Origin *;
 	}
 	{% endif %}