Request: a new gxadmin command that can disable/delete a user account

A handy gxadmin command to disable/delete a user account.

1. Accepts a domain (e.g., `@XXXX.YYY`) name and deletes all domain users.
2. Accepts a user `id` and deletes that specific user.
3. Accepts an email and delete that specific user.

No file, test, or command entry point is named in the issue. Start by locating how gxadmin commands handle user operations and how they access the Galaxy database, then confirm the expected behavior for domain, ID, and email inputs; done means the command safely performs the requested account deletion or disabling for each supported input.

Written by the indexing model from the issue text.