Rewrite email parser using ImapEngine, harden processing loop

Replace webklex/php-imap with directorytree/imapengine in the ticket
email parser. ImapEngine is pure PHP over sockets.

Parser improvements:
- Wrap per-message processing in try/catch so one malformed email
  can't abort the run; failures are flagged and logged with UID
- Query unseen + unflagged so previously-failed (flagged) messages
  are no longer re-processed on every cron run
- Skip vacation/auto-responder emails (RFC 3834) to prevent mail
  loops with the ticket auto-reply
- Cap messages per run (50) and attachment size (15MB); inline
  images over 2MB are stored as attachments instead of base64-embedded
  in ticket details
- Atomic lock file creation
- preg_quote() the ticket prefix in subject matching
- Dedupe CC watchers and exclude the sender
- Map webklex 'tls' encryption setting to STARTTLS for compatibility

NDR/DSN parsing now walks MIME parts via the underlying
zbateson parser instead of relying on attachment extraction.

plugins/vendor/symfony/mime/DraftEmail.php

@@ -0,0 +1,45 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime;
+
+use Symfony\Component\Mime\Header\Headers;
+use Symfony\Component\Mime\Part\AbstractPart;
+
+/**
+ * @author Kevin Bond <kevinbond@gmail.com>
+ */
+class DraftEmail extends Email
+{
+    public function __construct(?Headers $headers = null, ?AbstractPart $body = null)
+    {
+        parent::__construct($headers, $body);
+
+        $this->getHeaders()->addTextHeader('X-Unsent', '1');
+    }
+
+    /**
+     * Override default behavior as draft emails do not require From/Sender/Date/Message-ID headers.
+     * These are added by the client that actually sends the email.
+     */
+    public function getPreparedHeaders(): Headers
+    {
+        $headers = clone $this->getHeaders();
+
+        if (!$headers->has('MIME-Version')) {
+            $headers->addTextHeader('MIME-Version', '1.0');
+        }
+
+        $headers->remove('Bcc');
+
+        return $headers;
+    }
+}