✨(backend) add django-treebeard to allow tree structure on documents

We choose to use Django-treebeard for its quality, performance and
stability. Adding tree structure to documents is as simple as
inheriting from the MP_Node class.

src/backend/impress/settings.py

@@ -299,6 +299,7 @@ class Base(Configuration):
         "dockerflow.django",
         "rest_framework",
         "parler",
+        "treebeard",
         "easy_thumbnails",
         # Django
         "django.contrib.admin",