Fix HTML email section breaks

Summary: Fixes T6481. Provides better section handling.

Test Plan:
Test an email with multiple sections, multiple mail platforms.

{F231022}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6481

Differential Revision: https://secure.phabricator.com/D10811

src/applications/metamta/view/PhabricatorMetaMTAMailBody.php

@@ -79,8 +79,13 @@ final class PhabricatorMetaMTAMailBody {
 
   public function addHTMLSection($header, $html_fragment) {
     $this->htmlSections[] = array(
-      phutil_tag('div', array('style' => 'font-weight:800;'), $header),
-      $html_fragment,
+      phutil_tag(
+        'div',
+        array(),
+        array(
+          phutil_tag('strong', array(), $header),
+          phutil_tag('div', array(), $html_fragment),
+        )),
     );
     return $this;
   }