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.
Revert "Use default value adapters to fill metadata on mail."

This reverts commit d6b4718dbd272d0d28581bbaded8920aa06d9a60.

Conflicts:
	opengever/mail/mail.py
	opengever/mail/tests/test_mail_metadata.py
Make a lower case query to get the author by email (See http://stackoverflow.com/a/16573690/1599111).
Use date instead of date for mail related fields.