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/psr/http-message/README.md

@@ -0,0 +1,16 @@
+PSR Http Message
+================
+
+This repository holds all interfaces/classes/traits related to
+[PSR-7](http://www.php-fig.org/psr/psr-7/).
+
+Note that this is not a HTTP message implementation of its own. It is merely an
+interface that describes a HTTP message. See the specification for more details.
+
+Usage
+-----
+
+Before reading the usage guide we recommend reading the PSR-7 interfaces method list:
+
+* [`PSR-7 Interfaces Method List`](docs/PSR7-Interfaces.md)
+* [`PSR-7 Usage Guide`](docs/PSR7-Usage.md)