Merge remote-tracking branch 'nickvergessen/ticket/12518' into develop-ascraeus

* nickvergessen/ticket/12518:
  [ticket/12518] Add event to overwrite the cannot edit condition in posting.php
  [ticket/12518] Add event to overwrite the allow edit and delete conditions
[ticket/12518] Apply De Morgan to make the conditions easier reasable

PHPBB3-12518