Add opt out link to weekly digest email

[mjtribble]

The weekly digest email needs to have an unsubscribe link
Figma Design

weekly_digest/templates/emails/weekly_digest_events.html

Opt out of the weekly digest subscription by setting WeeklyDigestSubscription.subscribed = False

[dashcraft]

PATCH: /weekly_digests/:digest_id
set subscribed = "False"

this route needs to be a redirect route on the frontend