Add shell script to obfuscate DB fields

This (optionally) copies across the DB to a new one, then:

- Anonymises user names and emails in the users table;
- For access_limited documents:
  - Lorem-ipsums slugs in the documents tables;
  - Lorem-ipsums summary, body, and title in the edition_translations table.
Make sure we sanitise fact check requests
Sanitise titles of sensitive attachments
Sanitise filenames of attachments
Scrub unique keys in fact_check_requests