Allow mailing lists to be permanently deleted

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

Test Plan:
```
./bin/remove destroy PHID-MLST-nseux3r55escj573shsf
 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-MLST-nseux3r55escj573shsf (PhabricatorMetaMTAMailingList)

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

Destroying objects...
Destroying PhabricatorMetaMTAMailingList PHID-MLST-nseux3r55escj573shsf...
Permanently destroyed 1 object(s).
```

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9979

src/applications/mailinglists/storage/PhabricatorMetaMTAMailingList.php

@@ -1,7 +1,9 @@
 <?php
 
 final class PhabricatorMetaMTAMailingList extends PhabricatorMetaMTADAO
-  implements PhabricatorPolicyInterface {
+  implements
+    PhabricatorPolicyInterface,
+    PhabricatorDestructableInterface {
 
   protected $name;
   protected $email;
@@ -40,4 +42,16 @@ final class PhabricatorMetaMTAMailingList extends PhabricatorMetaMTADAO
     return null;
   }
 
+
+/* -(  PhabricatorDestructableInterface  )----------------------------------- */
+
+
+  public function destroyObjectPermanently(
+    PhabricatorDestructionEngine $engine) {
+
+    $this->openTransaction();
+    $this->delete();
+    $this->saveTransaction();
+  }
+
 }