[notifications] - Add notification model

[sachalifs]

It should have:

• id: ObjectId
• type: enum with the type of notification, e.g.: new topic, invited to forum, etc
• createdAt: timestamp
• seen: boolean indicating wether the notification was seen by the user
• data: Mixed containing any data the notification needs, like reference to other documents for instance
• user: ObjectId reference to the user
• frontend: boolean indicating wether it's a backend operation (send signup email) or a business logic notification (new topic published)