refactor removing user from group

This is in preparation for removing a user and their lists when the group owner blocks them.
Remove the user via models.group
Remove the lists via models.list

bookwyrm/models/group.py

@@ -85,6 +85,15 @@ class GroupMember(models.Model):
             group=join_request.group,
         )
 
+    @classmethod
+    def remove(cls, owner, user):
+        """remove a user from a group"""
+
+        memberships = cls.objects.filter(group__user=owner, user=user).all()
+        for m in memberships:
+            # remove this user
+            m.delete()
+
 
 class GroupMemberInvitation(models.Model):
     """adding a user to a group requires manual confirmation"""