Automatically extract domain mappings from CSV

There was a step often missed where we wouldn't include mappings from
the Whitehall source. This is now an explicit step in the
`generate-mappings.sh` script:

- It reads the domain name out of the canonical `sites.csv`
- It used `extract-mappings.rb` to generate a list of everything we know
  about in Whitehall relating to that domain
- The `fetch_$DEPT_mappings.rb` script should take `STDIN` as a source, which is
  what comes from the previous step.

See `tools/fetch_scotland_mappings.rb` for example usage.