Put cows and figlet bannners in <pre> in HTML mail bodies

Summary: Fixes T9538. Ref T9408. `cowsay` and `figlet` Remarkup rules are being mangled in HTML mail right now. Put them in <pre> to unmangle them.

Test Plan:
Sent myself a cow + figlet in mail.

Used `bin/mail show-outbound --id ... --dump-html > dump.html` + open that HTML file in Safari to preview HTML mail.

Saw linebreaks and monospaced formatting.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9538, T9408

Differential Revision: https://secure.phabricator.com/D14248

src/infrastructure/markup/interpreter/PhabricatorRemarkupCowsayBlockInterpreter.php

@@ -28,10 +28,16 @@ final class PhabricatorRemarkupCowsayBlockInterpreter
       ->setText($content)
       ->renderCow();
 
-    if ($this->getEngine()->isTextMode()) {
+    $engine = $this->getEngine();
+
+    if ($engine->isTextMode()) {
       return $result;
     }
 
+    if ($engine->isHTMLMailMode()) {
+      return phutil_tag('pre', array(), $result);
+    }
+
     return phutil_tag(
       'div',
       array(

src/infrastructure/markup/interpreter/PhabricatorRemarkupFigletBlockInterpreter.php

@@ -27,10 +27,16 @@ final class PhabricatorRemarkupFigletBlockInterpreter
 
     $result = $figlet->lineEcho($content);
 
-    if ($this->getEngine()->isTextMode()) {
+    $engine = $this->getEngine();
+
+    if ($engine->isTextMode()) {
       return $result;
     }
 
+    if ($engine->isHTMLMailMode()) {
+      return phutil_tag('pre', array(), $result);
+    }
+
     return phutil_tag(
       'div',
       array(