Changed the relation User to UserPreferences to a OneToOne bidirectional relationship, created functions to fetch subscribed users and unsuscribed users
Can now getSubscribedUsers, getUnsuscribedUsers and getUsersMissingFilter
Added defaultNotification for single user. Fixed error in NotificationManager where the method in filter was reset to first method only.
Can now send bulk notifications to all users
Each notificationKey now contains an array  subscriberRoles that the subscriber will need to have in order to subscribe to that notificationKey
small changes
NotificationKeys have now a isBulkable boolean field to avoid sending single notifications as bulkNotifications
Added function in notificationManager to check if a user has the required roles to receive a notification.
Notifications are now properly sent depending on the user roles