Rename the new relationship to extra_organisations to be less confusing

- Before, it was named organisations, which was confusing as there's
  also Site.organisation. The new name distinguishes the meaning of
  the relationship to be other organisations that can edit the site.
- The Site.organisation relationship is the organisation that owns
  the site. Bouncer uses the Site.organisation_id field, so instead of
  referring to the same thing in different ways in both Transition
  and Bouncer, we've created a new relationship and kept Bouncer's
  the same, as Bouncer doesn't need the new behaviour - it just needs
  to know who primarily owns the site.