Ensure that site-org relationships always reflect organisation slugs

- When an organisation decides that it doesn't want a
  sub-organisation editing mappings anymore, we can remove the
  sub-organisation's slug from organisation_slugs in the YAML and the
  import will remove the relationship. That way, we don't have to
  manually delete the rows from the database.
Improve test coverage

- When multiple organisations are added, then one is deleted from a
  site, the relationship for the other still exists.