Use an opt-out flow for comments in video, springify notifications and
notifications handlers

The NodeEvaluation makes it difficult to use a course node in the
learning path access concept. So it is refactored to a minimum extent,
so that it can covering the previous functionality.
As a replacement a new security callback and a new tree node type is
introduced.

OO-848: enhance the interface of course nodes to do a post copy processing different from the import one

OO-990: rework importation of course node with references, implement autowiring capabilities in controllers

--HG--
branch : bps