[ticket/12896] Add php event for admin notices

PHPBB3-12896
Merge branch 'ticket/forumhulp/12896' into develop-ascraeus

* ticket/forumhulp/12896:
  [ticket/12896] Add php event for admin notices
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12896] Add php event for admin notices