add scope to single model journey

The key to getting the context working was instance_exec to allow me to pass the class as an argument
I prefer the use of:

```
model_scope proc { |clasz| clasz.for_company(current_company.id) }
```

If we pass the controller instance and we need to call some protected method on the controller we'll not have access. An example is if the method :current_company is private.