[ticket/13015] Allow adding multiple notifications per user about the same item

It is impossible to send notification to user about item user already has been
notified. The limitation is hardcoded in notifications manager class.
To allow adding multiple notifications per user about the same item,
move the limitation to the notification type class to be able to set
corresponding notifications behavior on per-type notification basis.

<a href="https://tracker.phpbb.com/browse/PHPBB3-13015">PHPBB3-13015</a>.