Organisation#with_sites_managed_by_transition scope uses extra_sites

- Treat the FK relationship between a site and its org plus those in
  the many-to-many as functionally equivalent for the purposes of
  showing the organisations. In practice, this means that orgs like
  "Go Science" that don't have their own site will appear as an org
  with a site in the orgs list.
- This Cucumber spec tests whether sub-orgs with no actual sites, but
  editing abilities on other sites (i.e. GO-Science -> BIS), will
  show up in the list of organisations. Users looking to edit
  GO-Science's mappings on BIS might not necessarily know to look
  inside BIS.