fix(a11y): present post streams as feeds, setting busy when loading new pages (#3522)

framework/core/js/src/forum/components/PostStream.js

@@ -104,7 +104,11 @@ export default class PostStream extends Component {
       );
     }
 
-    return <div className="PostStream">{items}</div>;
+    return (
+      <div className="PostStream" role="feed" aria-live="off" aria-busy={this.stream.pagesLoading ? 'true' : 'false'}>
+        {items}
+      </div>
+    );
   }
 
   onupdate(vnode) {