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 <sig@odoo.com>
