a11y: Correct HTML heading level hierarchy (use h2, not h3)

Summary:
Several Phorge pages include `<h1>` and `<h3>` HTML elements but no `<h2>` elements.

* https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/Heading_Elements instructs to "nest headings without skipping levels"; see also its "Don't do this" section.
* https://www.w3.org/WAI/tutorials/page-structure/headings/ says that "Skipping heading ranks can be confusing and should be avoided where possible: Make sure that a <h2> is not followed directly by an <h4>, for example."

Closes T16124

Test Plan:
* Look at the HTML source of a Maniphest task, a Paste, a Differential change, a Diffusion commit. See that h3 elements are replaced by h2 elements.
* Look at http://phorge.localhost/phame/ which used h2 elements before, no change.
* Optionally, run an Accessibility check in Chromium Lighthouse.

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: mainframe98, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16124

Differential Revision: https://we.phorge.it/D26115

resources/celerity/map.php

···
   'names' => array(
     'conpherence.pkg.css' => '384ce2d9',
     'conpherence.pkg.js' => '020aebcf',
-    'core.pkg.css' => '58f1a8df',
+    'core.pkg.css' => 'd0eb7f8f',
     'core.pkg.js' => '83580d78',
     'dark-console.pkg.js' => '187792c2',
     'differential.pkg.css' => '4c951b31',
···
     'rsrc/css/phui/phui-document-pro.css' => 'b7b3d677',
     'rsrc/css/phui/phui-document-summary.css' => 'b068eed1',
     'rsrc/css/phui/phui-document.css' => 'a9154763',
-    'rsrc/css/phui/phui-feed-story.css' => 'a0c05029',
+    'rsrc/css/phui/phui-feed-story.css' => '1dd2e4c0',
     'rsrc/css/phui/phui-fontkit.css' => '1ec937e5',
     'rsrc/css/phui/phui-form-view.css' => '0c6c5f44',
     'rsrc/css/phui/phui-form.css' => '2884219c',
···
     'phui-document-summary-view-css' => 'b068eed1',
     'phui-document-view-css' => 'a9154763',
     'phui-document-view-pro-css' => 'b7b3d677',
-    'phui-feed-story-css' => 'a0c05029',
+    'phui-feed-story-css' => '1dd2e4c0',
     'phui-font-icon-base-css' => 'b7608e58',
     'phui-fontkit-css' => '1ec937e5',
     'phui-form-css' => '2884219c',

src/applications/transactions/response/PhabricatorApplicationTransactionResponse.php

···
     }
 
     $aural = phutil_tag(
-      'h3',
+      'h2',
       array(
         'class' => 'aural-only',
       ),

src/applications/transactions/view/PhabricatorApplicationTransactionCommentView.php

···
       ->appendChild($anchor)
       ->appendChild(
         phutil_tag(
-          'h3',
+          'h2',
           array(
             'class' => 'aural-only',
           ),

src/view/phui/PHUIFeedStoryView.php

···
 
   public function setPontification($text, $title = null) {
     if ($title) {
-      $title = phutil_tag('h3', array(), $title);
+      $title = phutil_tag('h2', array(), $title);
     }
     $copy = phutil_tag(
       'div',

src/view/phui/PHUITimelineView.php

···
       ),
       array(
         phutil_tag(
-          'h3',
+          'h2',
           array(
             'class' => 'aural-only',
           ),

webroot/rsrc/css/phui/phui-feed-story.css

···
   color: {$darkgreytext};
 }
 
-.phui-feed-story-bigtext-post h3 {
+.phui-feed-story-bigtext-post h2 {
   font-size: {$biggestfontsize};
   line-height: 18px;
   color: {$darkgreytext};