User manager as mediator



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No need to do the `parseJSON` stuff. instead of using the `response` method from  Alamofire you can use `responseJSON` like [here](https://github.com/Alamofire/Alamofire#response-json-handler) and [in my Firebase code here](https://github.com/flatiron-school/digijam/blob/master/Digijam/FirebaseAPI.swift#L37)