[MERGE][FIX] mail, mass_mailing: fix blacklist behavior and its use in mass mailing

This merge fixes blacklist mechanism, mixin and its use in mass mailing.
Some fixes are related to https://github.com/odoo/odoo/commit/2ff9b379ef80ba6a8744fb9702a8088284372ddd. Tests are added in a
new module that will gradually hold more complex tests for mass mailing.

This commit is related to task ID 33224 (original blacklist implementation
done for v12) and its PR #25966 as well as task ID 1889703 (tests and fixes)
and closes PR #27330.