Allow Phriction documents to be permanently deleted

Summary: Allow `PhrictionDocument` to be permanently deleted with `./bin/remove destroy`.

Test Plan:
Deleted a Phriction document with `./bin/remove` and verified that the database was in the expected state.

```
> ./bin/remove destroy PHID-WIKI-auj57rauigvcqvv5feh6
 IMPORTANT: OBJECTS WILL BE PERMANENTLY DESTROYED!

There is no way to undo this operation or ever retrieve this data.

These 1 object(s) will be completely destroyed forever:

    - PHID-WIKI-auj57rauigvcqvv5feh6 (PhrictionDocument)

    Are you absolutely certain you want to destroy these 1 object(s)? [y/N] y

Destroying objects...
Destroying PhrictionDocument PHID-WIKI-auj57rauigvcqvv5feh6...
Permanently destroyed 1 object(s).
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9976

src/applications/phriction/storage/PhrictionDocument.php

@@ -5,7 +5,8 @@ final class PhrictionDocument extends PhrictionDAO
     PhabricatorPolicyInterface,
     PhabricatorSubscribableInterface,
     PhabricatorFlaggableInterface,
-    PhabricatorTokenReceiverInterface {
+    PhabricatorTokenReceiverInterface,
+    PhabricatorDestructableInterface {
 
   protected $slug;
   protected $depth;
@@ -180,4 +181,26 @@ final class PhrictionDocument extends PhrictionDAO
   public function getUsersToNotifyOfTokenGiven() {
     return PhabricatorSubscribersQuery::loadSubscribersForPHID($this->phid);
   }
+
+
+/* -(  PhabricatorDestructableInterface  )----------------------------------- */
+
+
+  public function destroyObjectPermanently(
+    PhabricatorDestructionEngine $engine) {
+
+    $this->openTransaction();
+
+      $this->delete();
+
+      $contents = id(new PhrictionContent())->loadAllWhere(
+        'documentID = %d',
+        $this->getID());
+      foreach ($contents as $content) {
+        $content->delete();
+      }
+
+    $this->saveTransaction();
+  }
+
 }