VB-10: Create NotificationUser model

[alexciornii]

1. Add enums associated with the NotificationUser model:

    NotificationStatus:
        - new
        - seen
        - deleted

2. Create NotificationUser model as described below:

    NotificationUser:
       user:
          type: User
       status:
          type: NotificationStatus
       notification:
          type: Notification
       created_at:
          type: datetime
       required:
        - user
        - status
        - notification

[RomanCudreavtev]

Please asign on me