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 

Discuss:
- [ ] 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 `
- [ ] references on extracted documents: Do i have to implement anything? imho it's already done. 
I'd call this `settings` instead of `proxy` - much more readable.