Map E-Mail headers to Document metadata and set defaults

- [x] Init metadata on mail
- [x] Author handling
- [x] delivery_date special handling
- [x] Upgrade step 
- [x] Move title and message into the common fieldset.
- [x] Fix mail indexes. 

Discuss:
- [x] metadata file: metadata behavior has a `archive_file`. I guess it makes no sense to fill this with the same data too. Check definition for OGIP6 `●	Datei: Enthält die *.eml Datei mit der Originalnachricht (ggf. nach Konvertierung `
- [x] references on extracted documents: Do i have to implement anything? imho it's already done. 
This should first default to `field.default` (even if that's `None`) - see [`z3c.form.widget`](https://github.com/zopefoundation/z3c.form/blob/master/src/z3c/form/widget.py#L114). You should probably also [bind the field to the context.](https://github.com/zopefoundation/z3c.form/blob/master/src/z3c/form/widget.py#L111)