Add ability to assign reading list ownership to Metron user

[bpepple]

Summary

• Adds an "Assign to Metron" action on the reading list detail view that lets staff or members of the "reading list editor" group reassign ownership of a reading list to the Metron account
• The action is gated behind a confirmation page (matching the existing delete-confirmation style) and is a one-way transfer
• The button is hidden once a list is already owned by Metron, and is available independently of the existing owner-only action bar (add issues/edit/delete)

Closes #552