Leave project/group from email in 1 click

In https://gitlab.com/gitlab-org/gitlab-ce/issues/59421, we cut scope significantly because of required backend and UX.

We do not yet leave a project/group with 1 click. Currently we show a browser confirmation when you visit /:namespace?leave=1. With backend help we can ditch the confirmation box.

To do so we need each email to have a unique token available to it to authenticate an action like leaving a project/group.

We have some existing logic there around unsubscribing from notifications

See https://gitlab.com/gitlab-org/gitlab-ce/blob/master/app/controllers/sent_notifications_controller.rb

• DouweM