update group and list models

- remove GroupList model
- add a group foreign key value  to List model
- remove reference to lists in Group model

bookwyrm/models/__init__.py

@@ -21,7 +21,7 @@ from .relationship import UserFollows, UserFollowRequest, UserBlocks
 from .report import Report, ReportComment
 from .federated_server import FederatedServer
 
-from .group import Group, GroupList, GroupMember
+from .group import Group, GroupMember
 
 from .import_job import ImportJob, ImportItem