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 isn't a case insensitive query yet. You make the email extraced from the `From:` header lowercase (which ` extract_email()` already does). But we wan't to compare against the lowercased SQL column value. So it should be something like

```
from sqlalchemy import func
principal = session.query(User).filter(func.lower(User.email == email).first()
```