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
Configure main land registry site on transition tool
Merge pull request #598 from rjc123/newmappings

Configure main land registry site on transition tool
Change date of ukti transition in TTool app
Merge pull request #599 from rjc123/newmappings

Change date of ukti transition in TTool app
Add tools to check domain status pre-transition
Clarifications to investigation scripts, plus error control
Add tool to automatically create list of known sites
Configure additional alias for EA main domain
Add tool to get copies of the whitehall mappings files quickly