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/laravel/serializable-closure/src/Exceptions/InvalidSignatureException.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Laravel\SerializableClosure\Exceptions;
+
+use Exception;
+
+class InvalidSignatureException extends Exception
+{
+    /**
+     * Create a new exception instance.
+     *
+     * @param  string  $message
+     * @return void
+     */
+    public function __construct($message = 'Your serialized closure might have been modified or it\'s unsafe to be unserialized.')
+    {
+        parent::__construct($message);
+    }
+}

plugins/vendor/laravel/serializable-closure/src/Exceptions/MissingSecretKeyException.php

@@ -0,0 +1,19 @@
+<?php
+
+namespace Laravel\SerializableClosure\Exceptions;
+
+use Exception;
+
+class MissingSecretKeyException extends Exception
+{
+    /**
+     * Create a new exception instance.
+     *
+     * @param  string  $message
+     * @return void
+     */
+    public function __construct($message = 'No serializable closure secret key has been specified.')
+    {
+        parent::__construct($message);
+    }
+}