Send HTML email by default

Summary: Ref T10694. Switch default mode to HTML since it has a number of significant advantages and we haven't seen reports of significant problems.

Test Plan:
  - Switched preference to default (saw "HTML" in UI).
  - Sent myself some mail.
  - Got HTML mail.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15885

src/applications/metamta/storage/PhabricatorMetaMTAMail.php

@@ -634,7 +634,7 @@ final class PhabricatorMetaMTAMail
       }
       $mailer->setBody($body);
 
-      $html_emails = false;
+      $html_emails = true;
       if ($use_prefs && $prefs) {
         $html_emails = $prefs->getPreference(
           PhabricatorUserPreferences::PREFERENCE_HTML_EMAILS,

src/applications/settings/panel/PhabricatorEmailFormatSettingsPanel.php

@@ -76,7 +76,7 @@ final class PhabricatorEmailFormatSettingsPanel
       ? pht('Vary')
       : pht('Do Not Vary');
 
-    $html_emails_default = pht('Plain Text');
+    $html_emails_default = pht('HTML');
 
     $re_prefix_value = $preferences->getPreference($pref_re_prefix);
     if ($re_prefix_value === null) {