Replace socket.io with plain WebSocket

Replace socket.io with plain WebSocket using the ws library as it
doesn't really provide any useful tools over plain WebSocket when it
comes to implementing reliable communication that works across
reconnects.  The widespread browser support for WebSocket also means
it has little value to add for the web interface over WebSocket.

client.js

@@ -390,17 +390,19 @@ class InstanceConnection extends link.Link {
 	}
 }
 
-class SlaveConnector extends link.SocketIOClientConnector {
+class SlaveConnector extends link.WebSocketClientConnector {
 	constructor(slaveConfig) {
-		super(slaveConfig.get("slave.master_url"), slaveConfig.get("slave.master_token"));
+		super(slaveConfig.get("slave.master_url"));
 
 		this.id = slaveConfig.get("slave.id");
 		this.name = slaveConfig.get("slave.name");
+		this._token = slaveConfig.get("slave.master_token");
 	}
 
 	register() {
 		console.log("SOCKET | registering slave");
 		this.send('register_slave', {
+			token: this._token,
 			agent: 'Clusterio Slave',
 			version,
 			id: this.id,
@@ -412,8 +414,7 @@ class SlaveConnector extends link.SocketIOClientConnector {
 /**
  * Handles running the slave
  *
- * Connects to the master server over the socket.io connection and manages
- * intsances.
+ * Connects to the master server over the WebSocket and manages intsances.
  */
 class Slave extends link.Link {
 	// I don't like God classes, but the alternative of putting all this state