Allow structured destruction of Maniphest tasks

Summary:
Ref T4749. Ref T3265. Ref T4909.

  - Support ManiphestTasks in `bin/remove`.
  - Transactions need manual help for now.

Test Plan:
  - Destroyed tasks.
  - Examined log to verify task, transaction, and comment destruction.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3265, T4749, T4909

Differential Revision: https://secure.phabricator.com/D8944

src/applications/maniphest/storage/ManiphestTask.php

@@ -7,7 +7,8 @@ final class ManiphestTask extends ManiphestDAO
     PhabricatorTokenReceiverInterface,
     PhabricatorFlaggableInterface,
     PhrequentTrackableInterface,
-    PhabricatorCustomFieldInterface {
+    PhabricatorCustomFieldInterface,
+    PhabricatorDestructableInterface {
 
   const MARKUP_FIELD_DESCRIPTION = 'markup:desc';
 
@@ -282,4 +283,26 @@ final class ManiphestTask extends ManiphestDAO
     return $this;
   }
 
+
+/* -(  PhabricatorDestructableInterface  )----------------------------------- */
+
+
+  public function destroyObjectPermanently(
+    PhabricatorDestructionEngine $engine) {
+
+    $this->openTransaction();
+
+      // TODO: Once this implements PhabricatorTransactionInterface, this
+      // will be handled automatically and can be removed.
+      $xactions = id(new ManiphestTransaction())->loadAllWhere(
+        'objectPHID = %s',
+        $this->getPHID());
+      foreach ($xactions as $xaction) {
+        $engine->destroyObject($xaction);
+      }
+
+      $this->delete();
+    $this->saveTransaction();
+  }
+
 }