New test for existing list form. --ch11l016--

Conflicts:
	lists/tests/test_forms.py
placeholder for ExistingListItemForm --ch09l071--
ExistingListItemForm now inherits from ItemForm. --ch09l072--
Use validate_unique on instance to catch dupes. --ch11l018--
use our new dupe item error constant in tests. --ch11l049--
Change isinstances tests to new form. --ch11l050--
use new ExistingListItemForm in view. --ch11l051--

Conflicts:
	lists/views.py

Use new form in view. --ch11l052--
New unit test for ExistingListItemForm save. --ch11l053--
use standard django save for existinglistitemform. --ch11l054--