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
Fix name of a migration class to not rely on inflection

This migration was failing because the class name was relying on
inflection of 'TNA', but that isn't loaded here. This reverts the
name to 'Tna' so that it runs.
Merge pull request #417 from alphagov/fix_migration_class_name

Fix name of a migration class to not rely on inflection