Fix tagging content to world taxons
Fix tagging content to world taxons

The World taxons were removed from the Topic taxonomy [1], but this
has broken the tagging interface in Whitehall for users from the
Foreign and Commonwealth Office, so lets just add it back from the
perspective of Whitehall in the short term to fix this functionality.

1: https://trello.com/c/AwIIPzrU/51-remove-the-world-branch-from-the-topic-taxonomy

alphagov/whitehall,alphagov/whitehall,alphagov/whitehall,alphagov/whitehall