[ticket/13146] Allow changing the result of calling get_forums_visibility_sql

https://tracker.phpbb.com/browse/PHPBB3-13146

PHPBB3-13146

Fixed branch to push on #3198
This is wrong. This function is supposed to return "The appropriate combination SQL logic for topic/post_visibility" and not some arbitrary boolean that you've created and added to phpBB's logic just for your purposes. You should not be adding new logic to phpBB's core when adding events. Just add the event (because you can do whatever you want from within your own extension via the event, and do not need to spill it out into the core like this).