Preview campaign

[mgax]

Fixes https://github.com/wagtail/wagtail-newsletter/issues/31

After some back and forth, we've changed how campaign data is saved: we require the user to save a revision, and base the campaign off of that revision's content. This way it's unambiguous what the campaign content will be; there's no room for the user to forget to save their changes on the Wagtail Page and/or have another user overwrite their work before the campaign is saved.

Therefore, I've changed the behaviour of the newsletter_recipients and newsletter_subject fields: they are no longer specially persisted regardless of revisions; they are now part of the regular revision content. The newsletter_campaign field is still persisted: it's not editable by the user, and it's not helpful to have it overwritten when restoring a previous revision.

[mgax]

The preview page's header is pretty basic at the moment. I wonder what it would take to make it more snazzy, maybe like the history page's header.

---