Server non-scary page at matrix domain (take 2)

Fix for 12b65d8ccc.

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

@@ -107,6 +107,6 @@ server {
 	}
 
 	location / {
-		set $location "/_matrix/static/";
+		rewrite ^/$ /_matrix/static/ last;
 	}
 }