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...) 

Hey Brian,

I have updated the class to UserNotificationConstants because I think that we shouldn't mix it with UserNotificationDeliveryConstants because that's only for delivery type while this constants are for notification type.

I also added a last commit to use include them in UserNotificationDefinition although I personally like it more in UserNoticationConstants.

Thanks!