Rename main routes, and keep a legacy for the main unsubscribe route to
keep retro compatibility for existing mailings.
Task-2710804 (Mail: Clean MailThread API)
Task-2669037 (Mass Mailing: Subscription page code cleaning)
Prepares Task-2150462 (Mass Mailing: Unsubscribe flow refactoring)
Part-of: odoo/odoo#106568
