Preview service should put big "PREVIEW" banner on PDFs

[arfon]

Occasionally people will use the previews from http://whedon.theoj.org in ways we would rather they didn't, for example uploading them to preprint servers. While JOSS doesn't forbid this activity, the previews include nonsense strings such as 'Mickey Mouse et al.' in the citation string as placeholder text.

We should put some kind of banner on the PDF to make it clear these are previews, not real drafts.