Build notifications: keeps sending emails after deleting the account

[humitos]

In the way that email "Build notifications" works (https://docs.readthedocs.io/en/stable/build-notifications.html#email-notifications) right now, Read the Docs keep sending emails after the user has deleted their account. This is because email notifications are not associated to the users itself, but they are associated to a particular email (it could be the same as the registered by the user, but the data is duplicated and not associated).

So, when deleting the user account, all the email notifications are not deleted because they are not related to the user just deleted. Currently, to avoid this stage, the user should delete all their email notifications from all the projects before deleting their account if they want to stop receiving build notification emails 😞

[stsewd]

I guess we should also allow people to unsubscribe without having to log in, since anyone can add your email to any project.

[stsewd]

Or maybe even require confirmation for emails that aren't related to the user adding it before we send notifications to it.