Serve a non-scary page at the matrix domain

Fixes #18 (Github issue).

It would probably be better if we serve our own page,
as the Matrix one says:

"To use this server you'll need a Matrix client", which
is true, but we install Riot by default and it'd be better if we mention
that instead.

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

@@ -105,4 +105,8 @@ server {
 		client_max_body_size {{ matrix_synapse_max_upload_size_mb }}M;
 		proxy_max_temp_file_size 0;
 	}
+
+	location / {
+		set $location "/_matrix/static/";
+	}
 }