Allow for private group deletion in h admin interface

Currently, private groups can only be deleted by developers, and it's a complicated process. As described by Robert:

> It requires SSH-ing into an app server, opening a shell with full read/write access to the production database, and executing Python code to manipulate the database objects.

This is a problem because we cannot delete a user's account if they have created a private group where other users have annotated.

While group deletion for users will be handled by https://github.com/hypothesis/product-backlog/issues/532, we need a way to delete private groups in the h admin interface as well so we can comply with account deletion requests.

Start by reviewing the h admin interface and the Python-based private-group deletion process described in this issue. Compare the required behavior with product-backlog#532, then define how an authorized admin can delete private groups while supporting account-deletion requests; done means the deletion no longer requires SSH access or direct production-database manipulation.

Written by the indexing model from the issue text.