Feature/unsubscribe

This'll require you to add some stuff to the config
```json
{
  "...": "...",
  "mail": {
    "sendy": {
      "url": "url.to.sendy/withslash/",
      "key": "apikey-3y87fdshjf",
      "announcement_list": "announcement list id"
    }
  },
  "...": "..."
}
```

Tested locally but of course would appreciate someone else checking this out
No feature flag?