Introduce a one-click "unsubscribe" button in notification emails

There is a risk that notification emails get considered as spam.

See, for example, the rules concerning marketing emails sent to gmail users:
https://support.google.com/a/answer/81126?visit_id=638452602659076694-585339297&rd=1#subscriptions&zippy=%2Crequisitos-para-todos-os-remetentes%2Cconditions-requises-pour-envoyer-messages-ou-plus-par-jour

One thing that could help presenting these messages as legit, and to enhance the user experience in the same time, would be to add an "unsubscribe link", as well as the following headers:

List-Unsubscribe-Post: List-Unsubscribe=One-Click
List-Unsubscribe : <https://xwiki.org/unsubscribe/example>