Add self-service Unsubscribe alternative to /subscribed

Only shows when reached via a ?chain=&email= link (see portal's
self_transition), matching the link the newsletter workflow now
appends to every send.

questions/subscribed.yaml

@@ -12,3 +12,12 @@ alternatives:
         description: A few considered notes a year — nothing more.
       - name: Easy to leave
         description: Every note carries a genuine, one-click unsubscribe.
+
+  - name: Unsubscribe
+    description: >
+      Using the link from one of our emails? Confirm below and we'll
+      stop writing.
+    self_transition:
+      bucket: portal_subscribers
+      to: unsubscribed
+      label: Unsubscribe me