Make slug_exists_in_whitehall? more flexible

- This way, we can use it not just for whitehall_slug but for
  extra_organisation_slug (see transition for context).
Add all_slugs and missing_slugs methods

- Fetch all of the slugs (whitehall_slug and extra_organisation_slugs)
  from the site YAML in the first method, `all_slugs`.
- In `missing_slugs`, find organisation slugs that don't exist in
  whitehall.
Refactor `all_slugs` to use Ruby's .tap method

- This is better because it's clearer that it is going to return an
  array, and the whitehall_slug is going to be at the beginning.
Make `check_all_slugs` check both whitehall & extra_org slugs

- Change `missing` to be a hash so we're notified of more than one
  wrong slug.
- Change some of the YAML examples to be more descriptive.
Use a specific site YAML in the hope of fixing a failing CI test

- All the tests pass in dev, but one test is consistently failing on
  CI. It might be randomly ordering *.yml and picking a non-existent
  one, so set it to explicitly test ago.yml. Maybe this will fix it.
Merge pull request #597 from alphagov/check_extra_organisation_slugs

Make the whitehall:check_slugs rake task check both extra_organisation_slugs and whitehall_slug