add an audit event on the document audit trail

When a comment is posted , related directly or indirectly (i.e. as an answer of an another comment), we should add an audit entry on the document itself.

Category "Comments". Event name "Comment added". " A comment was added by ...." (+ storing ideally also the link to the comment so that it is leveraged by a UI layer after that).

We should also add at least Comment Removed. And ideally comment updated.