Fixed #19733 - deprecated ModelForms without 'fields' or 'exclude', and added '__all__' shortcut

This also updates all dependent functionality, including modelform_factory
 and modelformset_factory, and the generic views `ModelFormMixin`,
 `CreateView` and `UpdateView` which gain a new `fields` attribute.

tests/foreign_object/tests.py

@@ -322,6 +322,7 @@ class FormsTests(TestCase):
     class ArticleForm(forms.ModelForm):
         class Meta:
             model = Article
+            fields = '__all__'
 
     def test_foreign_object_form(self):
         # A very crude test checking that the non-concrete fields do not get form fields.