Fix a mail stamp issue with blocking reviewers

Summary: Revisions with blocking reviewers had this stamp built incorrectly, which cascaded into trying to use `array()` as a PHID. Recover so these tasks succeed.

Test Plan: Will deploy production.

Differential Revision: https://secure.phabricator.com/D19082

src/applications/differential/engineextension/DifferentialMailEngineExtension.php

···
       } else {
         $reviewers[] = $reviewer_phid;
         if ($reviewer->isBlocking()) {
-          $reviewers[] = $blocking;
+          $blocking[] = $reviewer_phid;
         }
       }
     }

src/applications/metamta/stamp/PhabricatorPHIDMailStamp.php

···
       return null;
     }
 
+    // TODO: This recovers from a bug where blocking reviewers were serialized
+    // incorrectly into the flat mail stamp list in the worker queue as arrays.
+    // It can be removed some time after February 2018.
+    foreach ($value as $key => $v) {
+      if (is_array($v)) {
+        unset($value[$key]);
+      }
+    }
+
     $viewer = $this->getViewer();
     $handles = $viewer->loadHandles($value);