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.
There's two points that @jennyd made here - one is that the implementation should change, which you've done. The other is that the name should be changed to match the existing validator. It should be `NotANationalArchivesURLValidator`, or similar.