[ticket/12559] Add forum setting to limit subforum legend

Squashed all commits, they were basically just typos and errors

PHPBB3-12559
[ticket/12559] Applied DeMorgan and removed unnecessary true:false

PHPBB3-12559
[ticket/12559] Updated copyright information

PHPBB3-12559
Merge pull request #4 from PayBas/ticket/12559

[ticket/12559] Add forum setting to limit subforums legend to direct children only