[ticket/12415] Use private message instead of "pm" accronym

https://tracker.phpbb.com/browse/PHPBB3-12415

PHPBB3-12415
Merge remote-tracking branch 'MGaetan89/ticket/12415' into develop-ascraeus

* MGaetan89/ticket/12415:
  [ticket/12415] Use private message instead of "pm" accronym