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
An alternative here would be:

```ruby
model_class.instance_exec(self, &scope)
```
the proc would then look like
```ruby
model_scope proc { |controller| for_company(controller.current_company.id) }
```

instead of

```ruby
model_scope proc { |clasz| clasz.for_company(current_company.id) }
```

I don't really have a preference either way.

_NB_
I did consider doing some method missing magic to make the proc operate in the context of the model class and route missing methods to the controller but realised that would be mental and may screw with existing active model method missing stuff.