[FIX] mass_mailing: have unsubscribe in multi lang
[FIX] mass_mailing: have unsubscribe in multi lang

Since 953a693df link not corresponding to a route are tought multilang
by default.

Thus in the mass mailing, if the user had not the default language on
the website he would have eg. a /fr_FR/unsubscribe_from_list placeholder
in the mail.

The placeholder is replaced by a real link pinpointing to the items the
given person would unsubscribe to, but having /fr_FR/ part prevented
that.

With this PR, there is a dummy route so /unsubscribe_from_list is
explicitely known as not multilang.

opw-1850696
closes #24912

Co-authored-by: Goffin Simon <b3ed2cf313e7546085c3c50622143ff31e467d23@odoo.com>

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo