Add ability to show users who haven't activated account

Some users are having trouble following up with account activation. Make it easier to find user accounts that have not been activated, so the admin user can follow up.

No files, tests, or entry points are named. Start by tracing the admin user-management flow and account-activation state, then define how unactivated accounts should be identified and verify that an administrator can find them for follow-up.

Written by the indexing model from the issue text.