[ticket/12861] Add event to modify the post, poster and attachment data

PHPBB3-12861
Merge pull request #2745 from nickvergessen/ticket/12861

[ticket/12861] Add event to modify the post, poster and attachment data

* nickvergessen/ticket/12861:
  [ticket/12861] Add event to modify the post, poster and attachment data
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12861] Add event to modify the post, poster and attachment data