Improve performance of notification dispatching by reducing right checks

Notification dispatching can be very slow on large instances when the security cache cannot be used efficiently (for example, because it is full) or right checks are slow because of custom access control rules that are expensive to evaluate. This is because, at the moment, notification dispatching checks for every dispatched event for every user a) if the user has access to the associated entity (document, space or wiki) and b) if the user should get a notification according to the notification filters. In many cases, users have access to the entity associated with the event but shouldn't be notified. The order should be inverted to first check if the user should get a notification and then check rights to avoid checking rights for most users. This will in particular help in wikis with many users who are rarely or never getting any notifications.