DifferentialCommentEditor::alterReviewers is now aware of differential.allow-self-accept setting

Summary: This allows users to add a revision's author as reviewer according Differential configuration using the 'Leap Into Action' form.

Test Plan: Tested on local install.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T1885

Differential Revision: https://secure.phabricator.com/D3682