Initialize metadata on mail.
Fill document_author metadata with fullname queried by email address (OGDS).
Upgrade step: initialize metadata on extisting mail instances
Init missing field values in existing mails. Test metadata upgrade step.
Implement setting default values for mail metadata.
Remove specific mail index methods (document_date, document_author, receipt_date, sortable_author).
Move title and message into the common fieldset / Move title at first position.