[IMP] digest: ease the way people can unsubscribe from a digest

Ease the way people can unsubscribe from a digest by adding information
to the email enabling a user agent to present a button to the user that
automate the unsubscription.

Technical notes:
To ease the way people can unsubscribe from a digest, we have followed the
advice of rfc8058 by adding the following header to the email:
- List-Unsubscribe: https_URL_to_unsubscribe
- List-Unsubscribe-Post: List-Unsubscribe=One-Click

The https_URL_to_unsubscribe
- must work with the POST method but not the GET. The GET version is disabled
to avoid unintended unsubscription that could be triggered by an anti-spam
accessing the URLs in the headers.
- must unsubscribe the user without any additional steps (one click)
- should contain an opaque identifier hard-to-forge to identify the list and
the user (our token fulfills that goal). The goal is to avoid that someone
could unregister someone else easily.
- must fulfill other conditions like no cookie, no redirection, ...

It requires also that List-Unsubscribe and List-Unsubscribe-Post be covered by
a DKIM signature.

Task-2800499

closes odoo/odoo#86934

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>