NotificationEventSignal Model Extensions

[cdparra]

• Add a OneToMany relationship with User that also contains the boolean attribute read, initialized to FALSE
• Create the NotificationEventSignalArchive model to periodically archive old signals

Obs: ideally, this model should map to more than one table like notification_event_signals_2017, notification_event_signals_2018 and so forth as we create new archives every certan period of time. One way to do it might be to use @SecondaryTables, so just for future, add a comment on how to use this annotation for future archival tables.