Redact non-officer faces in images

[sea-kelp]

lucyparsons#39

[sea-kelp]

To break down this task, this would probably involve:

1. Add a redacted_img_id field to the faces table and Face.redacted_image relationship to the Face model
2. Update everywhere that uses face.original_image to use face.redacted_image when available
3. Add a page similar to the face cropping page for logged in users to redact faces
4. Regenerate thumbnail if saving a redacted image with a thumbnail