Add unsubscribe link

app/mailers/emails/admin_notification.rb

 module Emails
   module AdminNotification
     def send_admin_notification(user_id, subject, body)
+      email = recipient(user_id)
+      @unsubscribe_url = public_unsubscribe_url(email: email)
       @body = body
-      mail to: recipient(user_id), subject: subject
+      mail to: email, subject: subject
     end
   end
 end
\ No newline at end of file

app/views/notify/send_admin_notification.html.haml

 = simple_format @body
+
+\----
+
+%p
+  Don't want to receive updates from GitLab administrators?
+  = link_to 'Unsubscribe', @unsubscribe_url
\ No newline at end of file

app/views/notify/send_admin_notification.text.haml

 = h @body
+
+\-----
+
+Don't want to receive updates from GitLab administrators?
+== Unsubscribe here: #{@unsubscribe_url}
\ No newline at end of file

app/views/public/unsubscribes/show.html.haml

 %h3.page-title Unsubscribe from Admin notifications
-%p
-  Don't want to receive any updates from your system administrator.
-  By clicking the confirmation button you'll no longer receive these notifications
 
 %hr
 = form_tag public_unsubscribe_path(@email) do