Allow extensions decide whether to add multiple notifications per user about the same item

Currently it is impossible to send notification to user about item user already has been notified. The code responsible is https://github.com/phpbb/phpbb/blob/develop-ascraeus/phpBB/phpbb/notification/manager.php#L384-L397
This can limit some extensions functionality, which may require sending multiple notifications per user about the same item (example: Thanks for posts ext which sends notifications to poster about "thanks" given by another users for the same post).

The suggestion is to move the limitation to the notification type class to be able to set corresponding notifications behavior on per-type notification basis.