Fix /.well-known/matrix/client for CORS

This is provoked by Github issue #46.

No client had made use of the well-known mechanism
so far, so the set up performed by this playbook was not tested
and turned out to be a little deficient.

Even though /.well-known/matrix/client is usually requested with a
simple request (no preflight), it's still considered cross-origin
and [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS)
applies. Thus, the file always needs to be served with the appropriate
`Access-Control-Allow-Origin` header.

Github issue #46 attempts to fix it at the "reverse-proxying" layer,
which may work, but would need to be done for every server.
It's better if it's done "upstream", so that all reverse-proxy
configurations can benefit.

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

@@ -43,6 +43,7 @@ server {
 		root {{ matrix_static_files_base_path }};
 		expires 1m;
 		default_type application/json;
+		add_header Access-Control-Allow-Origin *;
 	}
 
 	{% if matrix_corporal_enabled and matrix_corporal_http_api_enabled %}