🐛(backend) stop using add_sibling method to create sandbox document

In a past release we added a feature to create a sandbox document to a
newly created used. To create this sandbox document, we duplicate an
existing document and this duplicate is using the add_sibling method
with the "right" agument on this original document. Adding a sibling at
the right to a document involve moving right every root document created
after the original document, so the path of all this documents are
recalculated and changed. This can lead to the lost of some leaf in a
tree because to do this operation, multiple locks are created on the
database, creating lot of connection to the database and if the max
number connection to the database is reached or if the memory allocated
by the database is too hight, the database can close all connections
leading to inconsistent paths in the Document table.

src/backend/core/models.py

@@ -285,8 +285,7 @@ class User(AbstractBaseUser, BaseModel, auth_models.PermissionsMixin):
                     )
                     return
 
-                sandbox_document = template_document.add_sibling(
-                    "right",
+                sandbox_document = Document.add_root(
                     title=template_document.title,
                     content=template_document.content,
                     attachments=template_document.attachments,