Changed `session.current.type` to a more generic label,`session_step`.

This implies that the customer has a collection "screenviews" with a property called "session_step".
I think it's more likely the property would be called "screenname" or something like that.
I think there should be an extra step after we extract the screennames where we add a session step label, just like we do with the money and play type steps. That way it works no matter what property name you use for your "screenviews" or whatever else you might have in your data model.