Purpose
=======
Remove the "email_send" field on the <mail.channel> (mass_mailing named
on the JS side).
This feature will be introduced with a new model (<mail.group>) in a
new module in the next commit.
Remove the email notification support on the channel, so now the mail
channels work only by chat.
Remove the "subject" on the Discuss side because this was used only on
"email" channel.
Technical
=========
In the mail channel model we can drop the usage of the blacklist as well
as the usage of the "email_to" field. Those two features were mainly
used for mailing list and have no utility for "chat like" channel.
Links
=====
Task-2510267
See odoo/odoo/pull/71599
See odoo/enterprise/pull/19296
See odoo/upgrade/pull/2600
std-odoo
9183a86fde