NOTIFPLT-47:  Add NotificationState to NotifictionEntry to track things ...

...like New, Complete, and Favorited

https://issues.jasig.org/browse/NOTIFPLT-47

This PR is a re-boot of an earlier one and reflects the discussion there.
Consider adding a few convenience methods for the consumers; e.g. addState(NotificationState), removeState(NotificationState).  That would make code such as FavoriteNotificationServiceDecorator a little simpler as I suspect there will be many consumers making minor state modifications and would provide a central point for logging and recording state changes.