[FIX] digest: ease unsubscribe

Purpose of this commit is to ease usage of unsubscribe button for periodic
digests for non-admins. Users land on page that says "You have unsubscribed
from ..." and not on digest form view anymore.

It is done using a public route with a token, allowing to unsubscribe even
when being not logged, for example when receiving digest emails that are
unwanted.

Task-2641394 (Digest emails sending improvement)
Task-2582128 (Digest onboarding and usage improvement)

Part-of: odoo/odoo#79653
Co-authored-by: Thibault Delavallée <tde@odoo.com>