Support Custom Cover Pages

Summary

@wrobbins recently added support for user provided cover images:

The clients however are still very specific about what they send to the backend:

In order for users to take advantage of the backend feature we will need to update the clients.

[ ] epub-press-js can accept the additional metadata
[ ] epub-press-js has docs for these new params (added here )
[ ] Bonus: A field is added in epub-press-chrome for the extension.
- Additional fields should be hidden by default and only shown if expanded? Maybe we save if the user has expanded these extra fields and consistently show the fields between usages.
- This is a stretch - another issue to followup on this would be sufficient.