Update WebSocketService according to the new API

src/app/services/websocket.service.ts

@@ -19,7 +19,7 @@ function jsonWebsocketConnect(url: string, input: Observable<object>, protocols?
 @Injectable()
 export class WebSocketService {
   private uplink: QueueingSubject<object>;
-  public downlink: Observable<WebSocketMessage<undefined>>;
+  public downlink: Observable<WebSocketMessage>;
 
   constructor() {}
 
@@ -33,14 +33,14 @@ export class WebSocketService {
       wsproto + window.location.host + '/ws',
       this.uplink = new QueueingSubject<object>()
     ).messages.pipe(
-        map(message => <WebSocketMessage<undefined>> message),
+        map(message => <WebSocketMessage> message),
         share()
       );
     console.log('ws connected');
   }
 
-  public observeKey<T>(key: string): Observable<WebSocketMessage<T>> {
-    return this.downlink.pipe(filter(message => message.key === key));
+  public observeKey<T extends WebSocketMessage>(key: string): Observable<T> {
+    return this.downlink.pipe(filter(message => message.key.startsWith(key)), map(message => <T> message));
   }
 
   public send(key: string, data?: any, trigger?: any): void {