Script to move content from one topic to another

This is part of https://www.pivotaltracker.com/story/show/76471962

All artefacts tagged with the source topic are retagged replacing the
source topic with the destination topic.  If search updates for the
artefact are controlled by panopticon, a search update is triggered too.
Merge pull request #208 from alphagov/script-for-retagging-topics

Script to move content from one topic to another