Add a preference to disable ghost comments

Summary: Ref T7447. At least some users dislike this feature so strongly that they'd prefer not to have it at all.

Test Plan: Viewed ghosts; toggled preference, no more ghosts.

Reviewers: chad

Reviewed By: chad

Subscribers: yelirekim, epriestley

Maniphest Tasks: T7447

Differential Revision: https://secure.phabricator.com/D12704