"magic numbers" in message_parser.php/bbcode.php

Looking at includes/message_parser.php & includes/bbcode.php there seem to be "magic numbers" hardcoded for the bbcode_ids. I would suggest these are converted to be constants instead.

By doing this it would help reduce issues that may be introduced as more bbcode types are added (this would be especially true for mods that add not trivial bbcodes).

An diff file has been attached with the suggested changes.