Rename base for ProductCategoryFormSet

Currently, the base class that is used to construct the
`ProductCategoryFormSet` and the `ProductCategoryFormSet` (as returned by
inlineformset_factory) share the same name. This makes overriding
harder and is different from the suggested method in the [Django docs](https://docs.djangoproject.com/en/1.4/topics/forms/formsets/#custom-formset-validation).
As the base FormSet is never used in Oscar apart from using it in the
factory, I've renamed it to `BaseProductCategoryFormSet`, keeping it in
line with Django docs.