Disallow adding a mapping that redirects to a TNA URL

- 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.
Disallow editing a mapping to redirect to a TNA URL
Make the not-TNA validator check a parsed URL's host, not just a string

- This makes it consistent with how NationalArchivesURLValidator checks
  for a TNA host.
Remove new URLs that were set to TNA archive links

- For redirects with URLs matching TNA ones, make the type an archive,
  make the archive_url for the particular mapping equal to the new_url
  and then delete the new_url.
- For all other types of mapping (archives, unresolved), if they have a
  new_url matching TNA, blank it.

(Paired with Jenny on this.)
Merge pull request #407 from alphagov/disallow_redirects_to_tna

Disallow redirects to TNA