Backport: Dossier: Fixed start_date default value

Use a `datetime.date` object for a schema.Date field instead of `datetime.datetime`.
Backport: Added `checked_out_by` possibility to the `DocumentBuilder`.
Backport: Reworked the Dossier Base tests.
Backport: Dossier resolving: Fixed `its_all_closed` check, for nested subtasks.

Conflicts:

	docs/HISTORY.txt