Fix users who participate to a conversation do not receive notifications

When users use Nuxeo to collaborate on a document, it is important that they receive relevant notifications, otherwise  people are not informed other people are trying to collaborate on the doc. As such, implement the following rules to fix the functional bug:

• On the first comment set on a doc, subscribe automatically dc:creator of the document if he is not already subscribed directly on the doc (no need to fetch parents) and subscribe the user who sets the comment (comment:author)

• On following comments, subscribe the comment:author of the new comment if he hasn't subscribed yet and if it is the first time he sets a comment on this doc. Make sure to not re-subscribe the document creator as maybe she on purpose unsubscribed from the document since.