PERF: fix subtrees never being retained if "mark all as read" was clicked

I didn't realise, performance was terrible for people who have ever
clicked the "mark all as read" button!

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

@@ -35,7 +35,7 @@ export default class DiscussionListItem extends Component {
      */
     this.subtree = new SubtreeRetainer(
       () => this.props.discussion.freshness,
-      () => app.session.user && app.session.user.readTime(),
+      () => app.session.user && app.session.user.readTime().getTime(),
       () => this.active()
     );
   }