lists/forms.py ItemForm.save() works now correctly. All tests pass. End of Chapter 11, p. 210
Added functionallity that an item can be only added once to an list in lists/models.py. lists/test_models.py extended and restructured. Chapter 12, p. 217
Renamed last migrations file to: 0005_list_item_unique_together.py Chapter 12, p. 218