[FIX] mass_mailing: added _get_translation_frontend_modules_name

Before: When a user receive a mass_mailing and click on the
unsubscribe button, he arrives on the unsubscribe webpage where
the first sentence stay in english whatever the website language

Step to reproduce:
- Create a db with the email marketing and web module
- Create a mailing list with at least one recipient
- Send the mailing (catch the email with an email catcher, ex: mailhog)
- Click on the unsubscribe button

Now: The first sentence is in the website (or portal) language

opw-3538873

closes odoo/odoo#152579

X-original-commit: 6ba3dc277b9bf6b093eeefe889f20734ae4d135b
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>

addons/mass_mailing/models/__init__.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # Part of Odoo. See LICENSE file for full copyright and licensing details.
 
+from . import ir_http
 from . import ir_mail_server
 from . import ir_model
 from . import link_tracker

addons/mass_mailing/models/ir_http.py

@@ -0,0 +1,12 @@
+# Part of Odoo. See LICENSE file for full copyright and licensing details.
+
+from odoo import models
+
+
+class IrHttp(models.AbstractModel):
+    _inherit = "ir.http"
+
+    @classmethod
+    def _get_translation_frontend_modules_name(cls):
+        mods = super()._get_translation_frontend_modules_name()
+        return mods + ["mass_mailing"]