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