[PHP] UCP PM submission hook

Identifer: core.ucp_pm_submit

Location: ucp_pm_compose.php
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