LPS-43915 Support notifications framework for Blogs. I have added the logic to the SubscriptionSender to make it easier to apply the framework for the rest of the core portlets (wiki, message boards, DL...)
LPS-43915 Use constants. I didn't use UserNotificationDeliveryConstants because that's only for delivery (in other plugins we use this class because for the delivery type, but in this case it's about the notification type)
LPS-43915 Move constants to UserNotificationDefinition? Although for me it makes more sense to have it in UserNoticationConstants.
LPS-43915 SF
LPS-43915 Fix language keys
LPS-43915 Lang keys