Add migration to import taggings from panopticon.

DO NOT MERGE WITHOUT:

- [ ] Code review
- [ ] Coordination with these tickets: https://github.com/alphagov/publisher/pull/270 and https://github.com/alphagov/panopticon/pull/227

Results:
https://gist.github.com/elliotcm/a54550683e4af893c53d
![screenshot 2014-11-04 10 37 02](https://cloud.githubusercontent.com/assets/109225/4898311/3c7635e6-640f-11e4-9683-55bd5c9c8e50.png)