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/model_inheritance_regress/tests.py

@@ -418,6 +418,8 @@ class ModelInheritanceTest(TestCase):
         class ProfileForm(forms.ModelForm):
             class Meta:
                 model = Profile
+                fields = '__all__'
+
         User.objects.create(username="user_only")
         p = Profile.objects.create(username="user_with_profile")
         form = ProfileForm({'username': "user_with_profile", 'extra': "hello"},