Make category slugs dashboard-editable.

docs/source/releases/v3.0.rst

 - Several models were updated to define a default ordering, to avoid issues with inconsistent ordering of
   items in the dashboard and elsewhere. Database migrations are required for these changes.
 
+- Category slugs can now be edited via the dashboard.
+
 Dependency changes
 ~~~~~~~~~~~~~~~~~~
 

src/oscar/apps/dashboard/catalogue/forms.py

                                                   ('RelatedFieldWidgetWrapper',
                                                    'RelatedMultipleFieldWidgetWrapper'))
 
-CategoryForm = movenodeform_factory(
+
+BaseCategoryForm = movenodeform_factory(
     Category,
-    fields=['name', 'description', 'image', 'is_public'],
+    fields=['name', 'slug', 'description', 'image', 'is_public'],
     exclude=['ancestors_are_public'])
 
 
+class CategoryForm(BaseCategoryForm):
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        if 'slug' in self.fields:
+            self.fields['slug'].required = False
+            self.fields['slug'].help_text = _('Leave blank to generate from category name')
+
+
 class ProductClassSelectForm(forms.Form):
     """
     Form which is used before creating a product to select it's product class