Implement basic ingress filtering on controller

Validate messages received on the controller from hosts and control
nodes before handling or forwarding them.  This should prevent
performing actions that are not allowed by the role system and block
simple attacks such as spoofing the source address of a message.

plugins/global_chat/messages.js

@@ -5,6 +5,7 @@ class ChatEvent {
 	static src = ["control", "instance"];
 	static dst = "instance";
 	static plugin = "global_chat";
+	static permission = null;
 
 	/** @type {string} */
 	instanceName;