Allow public on list of subscribers

Summary: Fixes T7317, allows public to be set on this list controller.

Test Plan: Tested a list of subscribers on a logged in and logged out Diff.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7317

Differential Revision: https://secure.phabricator.com/D11801