Update user.model.ts

[SimCitee]

• What kind of change does this PR introduce?
  Bug fix
• What is the current behavior?
  Interface 'UserDocument' cannot simultaneously extend types 'User' and 'Document'.
  Named property '_id' of types 'User' and 'Document' are not identical.
• What is the new behavior (if this is a feature change)?
  Error is fixed
• Other information:
  The _id:any attribute needs to be changed to _id: mongodb.ObjectID;