Add Set<NotificationState> to NotificationEntry to track things like New, Complete, and Favorited

https://issues.jasig.org/browse/NOTIFPLT-47

This changeset introduces the concept of states to Notification objects and refactors the FAVORITE feature to become a state.
Only current states (at least that's the plan).  There could, however, be an auditing UI that allows privileged users to trace every (historic) state transition of a notification.