Normalise sites YAML

YAML rewritten according to Psych's idea of canonical

- removes indentation
- removes --- from end of files
site_orgs_augmenter should add slugs for all sites

(not just organisations)
Add Whitehall slug for the organisation which owns each site
Dumb, poor-performing whitehall slug checker
Bump gds-api-adapters
Add Site ability to check all slugs
Add Rake task whitehall:slug_check
Add Directgov and Business Link special cases

Don't check for their slugs - they've never been
in Whitehall.
Add walesoffice_cymru's slug as a special case
Add whitehall:slug_check to the build

(that Jenkins calls)
Deal with directgov_ and businesslink_ sites

Via Redirector::Site#never_existed_in_whitehall? being true
for DG and BL sites, but never for anything else.
Make whitehall_slug_check faster by lots
Add new_site rake task

* Takes abbr and whitehall_slug as arguments
* Won't create sites for non-existent slugs