Update the discussion's post relationship when a reply is added

js/forum/src/components/composer-reply.js

@@ -48,6 +48,7 @@ export default class ComposerReply extends ComposerBody {
         readTime: post.time(),
         readNumber: post.number()
       });
+      discussion.data().links.posts.linkage.push({type: 'posts', id: post.id()});
 
       // If we're currently viewing the discussion which this reply was made
       // in, then we can add the post to the end of the post stream.