StatusModel status managers don't have custom methods of a default manager

## Problem

Sometimes you might want to add a custom manager with custom queryset methods to a model that inherits `StatusModel`. (for example, via `.as_manager()` method of a queryset class):

```python
class CustomQuerySet(models.QuerySet):

def custom_qs_method(self):
return self.filter(feedback='')

class Consultation(StatusModel):
STATUS = Choices('PENDING', 'FINISHED')
feedback = models.TextField(default='')

objects = CustomQuerySet.as_manager()

```
Later, you might need to apply the methods to a queryset that filtered by a status value. I guess, it would be nice to use a manager for a specific status, provided by `StatusModel` (in this way we don't need to add `.filter(status=value)` each time or to define extra methods in custom managers). So, It would be nice to be able to do something like that:

```python
Consultation.FINISHED.custom_qs_method()
```

But currently it's not possible, because status manager is a `QueryManager` instance, and it doesn't inherit a custom methods of default manager. I propose to complement current implementation of the status managers, to include custom qs/manager methods to a status manager.

Start by reading StatusModel's status-manager setup and QueryManager, then compare how CustomQuerySet.as_manager() exposes methods. Add coverage for Consultation.FINISHED.custom_qs_method() and verify that the status manager retains custom queryset methods while applying the status filter.

Written by the indexing model from the issue text.