[ticket/12344] Add core.pm_submit_before to function submit_pm

PHPBB3-12344
Merge branch 'ticket/lucifer4o/12344' into develop-ascraeus

* ticket/lucifer4o/12344:
  [ticket/12344] Add core.pm_submit_before to function submit_pm
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12344] Add core.pm_submit_before to function submit_pm