new <comments> element is added to file metadata, it contains the URL to the comments virtual node and the count of comments (this way client knows if it make sense to request comments or not).
Fixed triggers in plugin XEP-0264 (return value was missing).
New trigger in plugin XEP-0329 to allow component to add metadata (used here for comments url).
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.
