Proxy other /_synapse endpoints to the client API

Besides /_synapse/admin, there are other things like
/_synapse/oidc, etc.

We should just proxy everything.

Fixes #534 (Github Issue).

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

@@ -150,7 +150,7 @@
 	}
 	{% endif %}
 
-	location /_synapse/admin {
+	location /_synapse {
 		{% if matrix_nginx_proxy_enabled %}
 			{# Use the embedded DNS resolver in Docker containers to discover the service #}
 			resolver 127.0.0.11 valid=5s;