Things to consider:
- Make this page open to public (but subscribe functionality only for logged-in users).
- Don't use radio buttons; most users wouldn't expect that changing a radio button would do something without submission.. we need to figure out something else
- Show the user a success message when subscribed.
- Increment subscriber count after subscription.
- Should only appear for JS users. Non-JS users should be shown a link to Special:Newsletter/id/subscribe.