Update wdtk-routes.rb to add ni and northernireland
Update wdtk-routes.rb to add ni and northernireland

Adds /ni and /northernireland redirects.

Per @garethrees suggestion use 'northern-ireland' instead of
'northernireland'.

https://github.com/mysociety/whatdotheyknow-theme/pull/718#discussion_r499582878

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