Record which user accepted on behalf of packages/owners reviewers

Summary:
Ref T12271. Don't do anything with this yet, but store who accepted/rejected/whatever on behalf of reviewers.

In the future, we could use this to render stuff like "Blessed Committers (accepted by epriestley)" or whatever. I don't know that this is necessarily super useful, but it's easy to track, seems likely to be useful, and would be a gigantic pain to backfill later if we decide we want it.

Test Plan: Accepted/rejected a revision, saw reviewers update appropriately.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12271

Differential Revision: https://secure.phabricator.com/D17537

resources/sql/autopatches/20170322.reviewers.04.actor.sql

···
+ALTER TABLE {$NAMESPACE}_differential.differential_reviewer
+  ADD lastActorPHID VARBINARY(64);

src/applications/differential/storage/DifferentialReviewer.php

···
   protected $reviewerStatus;
   protected $lastActionDiffPHID;
   protected $lastCommentDiffPHID;
+  protected $lastActorPHID;
 
   private $authority = array();
 
···
         'reviewerStatus' => 'text64',
         'lastActionDiffPHID' => 'phid?',
         'lastCommentDiffPHID' => 'phid?',
+        'lastActorPHID' => 'phid?',
       ),
       self::CONFIG_KEY_SCHEMA => array(
         'key_revision' => array(

src/applications/differential/xaction/DifferentialRevisionReviewTransaction.php

···
             ->setReviewerPHID($dst_phid);
         }
 
+        $old_status = $reviewer->getReviewerStatus();
         $reviewer->setReviewerStatus($status);
 
         if ($diff_phid) {
           $reviewer->setLastActionDiffPHID($diff_phid);
+        }
+
+        if ($old_status !== $status) {
+          $reviewer->setLastActorPHID($this->getActingAsPHID());
         }
 
         try {