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