Add typings for mongoose

[krtk6160]

From @samerbuna :

I don't think typeof User is a good idea but I see it used in the mongoose schema too, so maybe leave it for now and create a new issue to refactor all mongoose types to use interfaces extending mongoose documents.

[bodymindarts]

When we do this I would advocate a naming pattern like XxxRecord ie UserRecord to clearly associate these types with the persistence layer that will hopefully emerge. The concept of a plain User type is then free to be used for business layer logic.