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/Rfc2231Encoder.php

@@ -0,0 +1,50 @@
+<?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;
+
+use Symfony\Component\Mime\CharacterStream;
+
+/**
+ * @author Chris Corbyn
+ */
+final class Rfc2231Encoder implements EncoderInterface
+{
+    /**
+     * Takes an unencoded string and produces a string encoded according to RFC 2231 from it.
+     */
+    public function encodeString(string $string, ?string $charset = 'utf-8', int $firstLineOffset = 0, int $maxLineLength = 0): string
+    {
+        $lines = [];
+        $lineCount = 0;
+        $lines[] = '';
+        $currentLine = &$lines[$lineCount++];
+
+        if (0 >= $maxLineLength) {
+            $maxLineLength = 75;
+        }
+
+        $charStream = new CharacterStream($string, $charset);
+        $thisLineLength = $maxLineLength - $firstLineOffset;
+
+        while (null !== $char = $charStream->read(4)) {
+            $encodedChar = rawurlencode($char);
+            if ('' !== $currentLine && \strlen($currentLine.$encodedChar) > $thisLineLength) {
+                $lines[] = '';
+                $currentLine = &$lines[$lineCount++];
+                $thisLineLength = $maxLineLength;
+            }
+            $currentLine .= $encodedChar;
+        }
+
+        return implode("\r\n", $lines);
+    }
+}