Make links clickable in notes

Use auto_link gem to create links from URLs within edition notes.

![screen shot 2014-11-12 at 18 29 58](https://cloud.githubusercontent.com/assets/319055/5015923/f1120292-6a99-11e4-9823-904e8a480d48.png)

This feels like the wrong place to have the `html_safe` call.  I think this would be better within the helper.  That way it can be as focussed as possible.