This commit removes the strange selection box based on a magic flag and
some strange methods returning a string. Instead just allow to send mass
mailing on all models inheriting from mail.thread using the is mail
thread flag.
Various addons are updated to remove the _mail_mass_mailing class
attribute used to determine mass mailing capability.
We consider people could send a mass mailing on every model inheriting
from mail.thread. It makes no sense to limit it to a given set of addons.
Yannick Tivisse
51b5ee7c3c