Access rights on messages are derived from the
access rights on the documents they are attached
to. Due to the karma-based nature of the forum
access rights, these do not automatically reflect
on messages, because they are not implemented as
access rules.
The check_mail_message_access() needs to be
overriden to achieve the same effect.
+ allow calling super().check_mail_message_access()
from new API (useful in forward-port)