[ticket/12690] Add core.submit_pm_after event

Add core.submit_pm_after to funtion submit_pm.
Event will return just submited msg_id. It should be plased
just before return $data['msg_id'];

Justification:
Using only core.submit_pm_before does not allow to
follow up after message submission.
The message ID is recieved at DB submission.
Some times we need the MSG_ID as identifier

PHPBB3-12690