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).
thumbnails are saved in common cache, and sent blindly on request.
There is no access check for thumbnails yet, which is a security/privacy issue, but mitigated by the fact that content id must be known to retrieve the file.
Proper access check is planed.
