[ticket/12344] UCP PM submission hook

Add core.pm_pre_submit to funtion submit_pm

There is event request by EXreaction in the forum. As
I need it for few other extensions I think this fulfills it.

PHPBB3-12344
[ticket/12344] UCP PM submission hook

Add core.pm_pre_submit to funtion submit_pm

There is event request by EXreaction in the forum. As
I need it for few other extensions I think this fulfills it.

PHPBB3-12344
Merge branch 'develop' of https://github.com/lucifer4o/phpbb into develop