Topic retagging

Rake task to allow retagging of topics in whitehall

https://www.pivotaltracker.com/story/show/83304978


Probably best to hold off on this until https://github.com/alphagov/whitehall/pull/1874 is merged. This worker will no longer accept a hash as an argument because that causes issues with the marshalling and unmarshalling of the arguments performed by Sidekiq.