Add map directive to the base of nginx.conf

This needs to be added for WebSocket upgrades to work properly (see doc:
http://nginx.org/en/docs/http/websocket.html)

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

@@ -45,6 +45,11 @@ http {
 	keepalive_timeout 65;
 
 	#gzip on;
+	{# Map directive needed for proxied WebSocket upgrades #}
+	map $http_upgrade $connection_upgrade {
+		default upgrade;
+		''      close;
+	}
 
 	include /etc/nginx/conf.d/*.conf;
 }