Dossier close:  Fix `is all closed ` check.

When resolving a dossier with nested tasks, the `is all closed` doesn't work.

So I dropped the wrong depth limit in the query and refactored the whole method. Besides i reworked the complete dossier base tests. Additionally I changed the wrong start_date default value, because it broke the tests.

@lukasgraf could you take a look?