Unsubscribe link should appear on every email with link to unsubscribe form

[cjmabry]

• Each email should have an link that says Unsubscribe at the bottom
• This should take the user to a page with a pre-filled email address and an "Unsubscribe" button
• When users submit, they should be opted out of future emails
• The reason this can't just be a link that opts people out on click is because many email clients pre-fetch links for security scans, and a simple click would potentially inadvertently opt people out

For now, can create a signup form an manually maintain opt-outs, but this would be a v1.0 requirement.