remove list from group when changing curation

Allows 'group' to be blank when saving a list.
Removes the 'group' field when saving a list with curation other than 'group' - this stops the list "sticking" to a group after it is changed from group curation to something else.

bookwyrm/models/list.py

@@ -37,7 +37,8 @@ class List(OrderedCollectionMixin, BookWyrmModel):
     group = models.ForeignKey(
         "Group",
         on_delete=models.CASCADE,
-        null=True
+        default=None,
+        blank=True
     )
     books = models.ManyToManyField(
         "Edition",