feat(seo): Use h2 header for discussions on discussions list (#3731)

* Use H2 header for discussions on discussions list.

* Put `DiscussionListPane` in `aside` tag to indicate that its content should not be treated as part of discussion content.

* Update markup for views generated by PHP.

framework/core/views/frontend/content/discussion.blade.php

@@ -1,15 +1,15 @@
 <div class="container">
-    <h2>{{ $apiDocument->data->attributes->title }}</h2>
+    <h1>{{ $apiDocument->data->attributes->title }}</h1>
 
     <div>
         @foreach ($posts as $post)
-            <div>
+            <article>
                 @php $user = ! empty($post->relationships->user->data) ? $getResource($post->relationships->user->data) : null; @endphp
                 <h3>{{ $user ? $user->attributes->displayName : $translator->trans('core.lib.username.deleted_text') }}</h3>
                 <div class="Post-body">
                     {!! $post->attributes->contentHtml !!}
                 </div>
-            </div>
+            </article>
 
             <hr>
         @endforeach

framework/core/views/frontend/content/index.blade.php

@@ -1,7 +1,7 @@
 @inject('url', 'Flarum\Http\UrlGenerator')
 
 <div class="container">
-    <h2>{{ $translator->trans('core.views.index.all_discussions_heading') }}</h2>
+    <h1>{{ $translator->trans('core.views.index.all_discussions_heading') }}</h1>
 
     <ul>
         @foreach ($apiDocument->data as $discussion)