Provide safe exports of the production database for local development testing/debugging

[StevenMaude]

Having access to a copy production database would be useful to access production data for testing or debugging.

However, we should not include any potentially sensitive data (whether sensitive due to security concerns — for example, session data — or privacy concerns — for example, personally identifiable information).

Sensitive data should exist in production only.

This probably involves:

1. reviewing database columns to determine which are sensitive
2. devising a way to limit, remove or replace sensitive data

This relates to opensafely-core/job-server#1201.