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.
It might be a bit clearer if you used addition and subtraction on arrays here instead, eg.

```
artefact.specialist_sector_ids - [source_topic_id] + [dest_topic_id]
```