delete groups

Whoops, forgot to add this functionality earlier.

- allow owner to delete a group
- change all group lists to closed curation with group=False when group deleted

bookwyrm/models/list.py

@@ -32,7 +32,7 @@ class List(OrderedCollectionMixin, BookWyrmModel):
     )
     group = models.ForeignKey(
         "Group",
-        on_delete=models.PROTECT,
+        on_delete=models.SET_NULL,
         default=None,
         blank=True,
         null=True,