Fix wrong subclass in ConfigIgnore controller

Summary: Fixes T6436. We subclass the wrong controller and miss the admin-only check.

Test Plan: Ignored / unignored set up issues.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6436

Differential Revision: https://secure.phabricator.com/D10765