component file sharing: comments handling first draft:

comments use a minimal pubsub service which create virtual nodes for each files. A pubsub request to org.salut-a-toi.file_comments/[FILE_ID] allow to handle comments in classic way.
Permissions are the same as for files (i.e. if an entity can see a file, she can comment it).

src/plugins/plugin_xep_0329.py

@@ -75,7 +75,6 @@ class ShareNode(object):
         self.type = type_
         self.access = {} if access is None else access
         assert isinstance(self.access, dict)
-        self.persistent = False
         self.parent = None
         if parent is not None:
             assert name