AttributeError: 'NoneType' object has no attribute 'pubid' on `admin/groups`

https://sentry.io/organizations/hypothesis/issues/1071050977/?project=37293&referrer=slack

The admin-edit-group page is not meant to be used to edit private groups (and I don't believe links exist for private groups that go to this page), but we should fix the `admin-groups` page such that it doesn't throw a server error if it's hit for a private group.

The error is raised because the view assumes the presence of an organization for the group in question. Private groups do not belong to organizations.

Start by reproducing the admin-groups page for a private group and inspect the view handling that page, focusing on its assumption that the group has an organization. Done means the page no longer raises a server error when opened for a private group.

Written by the indexing model from the issue text.