mobiledoc_revisions() {
        return this.hasMany('MobiledocRevision', 'post_id');
    }
Store prev + latest mobiledoc content, because we have decided against a migration, which
iterates over all posts and creates a copy of the current mobiledoc content.

Reasons:
  - usually migrations for the post table are slow and error-prone
  - there is no need to create a copy for all posts now, because we only want to ensure
    that posts, which you are currently working on, are getting a content backup
  - no need to create revisions for existing published posts

The feature is very minimal in the beginning. As soon as you update to this Ghost version,
you