Fix a document 404ing with a redirect

www.agileplannerapp.com/boards/173808/cards/5678

it seems 2 editors started working on separate
drafts for the same content, hence there were
duplicate documents. the content lives at the
url that contains the slug with a sequence
number. hence redirecting to that location.
Merge pull request #1680 from alphagov/fix-document-404ing

Fix a document 404ing with a redirect