Make manager available in RunPython

[ptrstn]

Hello,

first of all thank your for this django package. I am in the process of replacing my own Category models with this package.

I was trying to write a data migration, but ran into a problem when trying to access the object manager in the Category model

NewCategory = apps.get_model('categories', 'Category')
...
NewCategory.objects.create(name=old_category.name)

caused following Error message:

AttributeError: type object 'Category' has no attribute 'objects'

I found out that in order to use a custom manager in RunPython data migrations you have to set the use_in_migrations attribute to True, as described here: https://docs.djangoproject.com/en/1.11/topics/migrations/#model-managers

In your package it would mean

class CategoryManager(models.Manager):
    """
    A manager that adds an "active()" method for all active categories
    """
    use_in_migrations = True

    def active(self):
        """
        Only categories that are active
        """
        return self.get_queryset().filter(active=True)

and maybe even the same to the CategoryRelationManager.

If you agree with this proposal, I could write a pull request if you wish.