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/views/__init__.py

@@ -45,6 +45,7 @@ from .group import (
     Group,
     UserGroups,
     FindUsers,
+    delete_group,
     invite_member,
     remove_member,
     accept_membership,