Disallow redirects to TNA

- We came across some redirects to TNA, instead of archives. This stops
  users from adding any URL beginning with
  http://webarchive.nationalarchives.gov.uk/ as a redirect, and editing them to add a redirect pointing to the same URL.
This validator effectively does the opposite of [NationalArchivesURLValidator](https://github.com/alphagov/transition/blob/master/app/validators/national_archives_url_validator.rb), but their names are different enough to obscure that a bit. They also work a bit differently: the other one parses the URL and checks the host explicitly, whereas this one only checks whether the hostname appears in the string. I think we should be consistent and check the host explicitly.