bug #4783 : Fix the notification to the users that have not enough right accesses to the commented content.

A new method is added to the SilverpeasContent interface: canBeAccessed(). This method is to verify a specified user has enough rights to access the content. Each concrete content must implement their way to verify the access right of a user by using the correct access controllers.

The comment notification service uses now this new method to verify a user can be recipient of the notification. In order to be notified, a user must have the privileges to access the commented content.

Don't forget to merge also the same branch in Silverpeas-Components.