MDL-67544 mod_forum: Correct aria-labelledby node

mod/forum/templates/forum_discussion_post.mustache

@@ -36,7 +36,7 @@
     data-region="post"
     data-target="{{id}}-target"
     tabindex="0"
-    aria-labelledby="post-header-{{id}}"
+    aria-labelledby="post-header-{{id}}-{{uniqid}}"
     aria-describedby="post-content-{{id}}"
 >
     <!-- The firstpost and starter classes below aren't used for anything other than to identify the first post in behat -->
@@ -48,7 +48,7 @@
         {{#isfirstunread}}<a id="unread" aria-hidden="true"></a>{{/isfirstunread}}
 
         <div class="d-flex flex-column w-100"  data-region-content="forum-post-core">
-            <header  class="mb-2 header row d-flex">
+            <header id="post-header-{{id}}-{{uniqid}}" class="mb-2 header row d-flex">
                 {{^isdeleted}}
                     {{#author}}
                         <div class="mr-2" style="width: 45px;">