[PHP] UCP PM submission hook
Before // Store message, sync counters
Parameters: Ideally, $pm_data, which is created after that line (but could be moved higher to be sent to the extension), $error, $submit, $preview, $refresh, $message_parser.
Explanation: Allows extensions to do things with PMs before they can be submitted (such as check for spam content).
Event requested by EXreaction