[IMP] mail: do not send email on join/leave mailing channel

Before this commit, when a user joined, left, or was invited to a
channel with "Send by email", it sent an email notification to every
member.

This commit disables email notification on subscribe/unsubscribe on
a channel, while still showing messages on join/leave.

Task-Id 2003281

closes odoo/odoo#35142

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>