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