Don't show comment draft state to non-object-owners

Summary: Fixes T7720. We currently leak the "draft" state of checkboxes; never treat checkboxes as drafts if you can't mark them.

Test Plan: Checked a box, reloaded page in other browser. Previously, the draft state partially propagated. Now, it no longer does.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T7720

Differential Revision: https://secure.phabricator.com/D12262