Expose "Abandon Revision" to non-authors with a config flag.

Summary: Fixes T4720. Allows non-authors to permanently reject a differential by exposing the "Abandon Revision" action via a configuration flag.

Test Plan: {F161434}

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T4720

Differential Revision: https://secure.phabricator.com/D9306