Implment chat
* src/board.php: add chat box
* css/chat.css: New file. CSS for chat box.
* js/chat.js: New file. JS for chat box.
* src/chat.php: New file. PHP for chat box.
* src/defines.php: Update to include chat files.
* src/session.php: Rename some cookies.
* src/titledbox.php: Add ID for titled box.
Finish chat implementation.
* css/chat.css: small tweaks, add online users field.
* js/chat.js: support new status message types, cookies for
unread messages, online users count.
* src/chat.php: add online users field.
* thread.php: add chat module.
Temporarily disable chat connection.