Preview translated editions via the main preview button

Let the user pick from available translations when using the main preview button:

![preview-translations](https://f.cloud.github.com/assets/74812/1684450/46c22c4e-5dcb-11e3-9a3e-63bd73388b42.png)

Also fixes an issue with saving empty strings instead of nils which meant the attachments weren't matched by `where(locale: [nil, …])`

Follow up for https://www.pivotaltracker.com/story/show/52384573