added Atmo prefix to User model class name

I found this model class lack namespace. For instance, UserKey uses namespace for the User model class. So I thought I add it here - my local tests shown it works. @nowakowski @mkasztelnik  Is it just a simple omission or the namespace was absent for a reason?
Align the parameters of a method call if they span more than one line.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.