Topic retagging

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

We want to update editions which are in draft state, etc, too.  Also, we only want to get slugs out of this, and we only want to get each slug once. I think the easiest way to do this is something like:

    Edition.joins(:specialist_sectors, :document).where(specialist_sectors: { tag: 'oil-and-gas/licensing' }).uniq.pluck(:slug)