Move an .html_safe to its helper
Add independent Whitehall import

* Redirector abbrs are temporarily pointed to whitehall_slug
  to satisfy uniqueness constraints
Get all mention of orgs out of SiteYamlFile

We used to infer orgs, now we don't, so SiteYamlFile
can finally deal with its only concern.
Stop trying to tie orgs to redirector abbrs

* (set redirector_abbr to whitehall_slug for now, as URLs are routed this way)
* Assume that orgs will be imported before sites and let sites find orgs by their Whitehall slugs
* OrgsSitesHosts becomes a tiny class that imports orgs, *then* sites
  (and by extension, hosts)
* Tidy up the awful mock blocks
* abridge the test data for whitehall orgs
Directgov/Business Link special cases

DG and BL never existed in Whitehall, which means we have to pretend
as if they did.
Set managed_by_transition appropriately
Fix import:orgs_sites_hosts (bad exception reference)
Remove Organisation#redirector-abbr

* Fix the features
* Fix the specs
* Fix the rake tasks