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
Test that script can run

If the schema changes this test will fail, which will alert us.
Scrub supporting pages
Signon DB isn't being scrubbed, so keep users
Lipsum all matching editions, not just latest

Based on feedback from @h-lame
Merge pull request #319 from alphagov/sanitise-database

Script to copy and sanitise the Whitehall database

https://www.pivotaltracker.com/story/show/40325147