Validate the host part of the hostnames added to transition

- Use URI.parse, making this essentially the same as the redirector
  change in https://github.com/alphagov/redirector/pull/608. As
  fallout from using URI.parse, change the site abbreviations from
  containing underscores to containing dashes in the test factories,
  because URI.parse does not handle hostnames with underscores in
  them. This means also changing the test data where site
  abbreviations with underscores are referenced, so they all still
  pass.