URL encode the opt-in-message anchor

Some browers won't redirect the hash if it's not encoded, Eg.

```
curl -i 'http://www.theguardian.com/preference/platform/mobile?page=http%3A%2F%2Fwww.theguardian.com%2Fuk%3Fview%3Dmobile%23opt-in-message'
```

/cc @rich-nguyen @mattosborn take note :)