Only include redirects in sitemaps

It's not helpful to have entries which point at 404s or 410s. It also meant
that rendering took longer and we were more likely to reach the 50k URL limit
for sitemaps.

This more or less copies the behaviour of the Redirector.
Merge pull request #20 from alphagov/only_include_redirects_in_sitemap

Only include redirects in sitemaps