Change subscription status in admin to radio field

The current admin change form for a subscription is a bit unintuitive since both 'Subscribed' and 'Unsubscribed' are rendered as individual checkboxes, although only one can be checked at a time.

Pull request turns this into a radio button field labeled 'Status' with 'Subscribed' and 'Unsubscribed' as the two options.

No functionality change, just a more intuitive experience in the admin area.