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/Encoder/Base64Encoder.php

@@ -0,0 +1,41 @@
+<?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\Encoder;
+
+/**
+ * @author Chris Corbyn
+ */
+class Base64Encoder implements EncoderInterface
+{
+    /**
+     * Takes an unencoded string and produces a Base64 encoded string from it.
+     *
+     * Base64 encoded strings have a maximum line length of 76 characters.
+     * If the first line needs to be shorter, indicate the difference with
+     * $firstLineOffset.
+     */
+    public function encodeString(string $string, ?string $charset = 'utf-8', int $firstLineOffset = 0, int $maxLineLength = 0): string
+    {
+        if (0 >= $maxLineLength || 76 < $maxLineLength) {
+            $maxLineLength = 76;
+        }
+
+        $encodedString = base64_encode($string);
+        $firstLine = '';
+        if (0 !== $firstLineOffset) {
+            $firstLine = substr($encodedString, 0, $maxLineLength - $firstLineOffset)."\r\n";
+            $encodedString = substr($encodedString, $maxLineLength - $firstLineOffset);
+        }
+
+        return $firstLine.trim(chunk_split($encodedString, $maxLineLength, "\r\n"));
+    }
+}