UserNotificationEvent table needs to be split into NotificationEvent and UserNotificationEvent

• Type: Story
• Status: Closed
• Priority: Minor
• Resolution: Completed
• Affects Version/s: None
• Fix Version/s: 7.0.0 M1
• Component/s: Collaboration, Collaboration > API > Notifications
• Labels: None
• Epic/Theme:
  • social_office

We need to define a new table NotificationEvent to extract common data for an event. And UserNotificationEvent refers to NotificatonEvent for the data. This change will be applied to 7.0.