♻️(backend) refactor list view to allow filtering other views

the "filter_queryset" method is called in the middle of the
"get_object" method. We use the "get_object" in actions like
"children", "tree", etc. which start by calling "get_object"
but return lists of documents.

We would like to apply filters to these views but the it didn't
work because the "get_object" method was also impacted by the
filters...

In a future PR, we should take control of the "get_object" method
and decouple all this. We need a quick solution to allow releasing
the hierchical documents feature in the frontend.

src/backend/core/tests/documents/test_api_documents_children_create.py

@@ -1,5 +1,5 @@
 """
-Tests for Documents API endpoint in impress's core app: create
+Tests for Documents API endpoint in impress's core app: children create
 """
 
 from uuid import uuid4

src/backend/core/tests/documents/test_api_documents_children_list.py

@@ -1,5 +1,5 @@
 """
-Tests for Documents API endpoint in impress's core app: retrieve
+Tests for Documents API endpoint in impress's core app: children list
 """
 
 import random