[REF] mail: rename notification methods and make them private

Purpose of this commit is to perform a light cleaning of the email
notification process. It renames method by prefixing them by _notify
in order to easily spot them. It also makes them private because they
should not be called outside of other methods. Indeed those methods
are technical and do not provide useful information to external
people.

Future commits will probably improve the notification process in order
to clean it and try to reduce its query count.

This commit perform the following renaming

 * message_get_email_values -> _notify_specific_email_values
 * message_get_recipient_values -> _notify_email_recipients
 * _message_notification_recipients -> _notify_classify_recipients
 * message_get_reply_to -> _notify_get_reply_to