Differential - add a configuration option to allow any user to close a revision that has been accepted

Summary: this is useful for certain workflows, typically where the reviewer is a gatekeeper of sorts who does the acutal commit. Special thanks to D2900 which made this relatively brain-dead to code up.

Test Plan: set to "true" in my local development environment and verified test user "xerxes" could close my stuff

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1732

Differential Revision: https://secure.phabricator.com/D3398