✨(back) create a cors proxy fetching docs external resources

When exporting a document in PDF and if the doc contains external
resources, we want to fetch them using a proxy bypassing CORS
restrictions. To ensure this endpoint is not used for something else
than fetching urls contains in the doc, we use access control and check
if the url really exists in the document.

src/backend/core/models.py

@@ -793,6 +793,7 @@ class Document(MP_Node, BaseModel):
             "children_list": can_get,
             "children_create": can_update and user.is_authenticated,
             "collaboration_auth": can_get,
+            "cors_proxy": can_get,
             "descendants": can_get,
             "destroy": is_owner,
             "favorite": can_get and user.is_authenticated,