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.
Add Transition::Import::Sites

Two purposes: importing sites and looking them up
by whitehall_slug
Make Import::Organisations refer to sites

(only gets the redirector_abbr)
* 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