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