CB Paid Subscriptions

If the subscription creation payment processing handling is reloaded (e.g. user reloading for whatever reason) WHILE the payment is currently processing (before basket state could change) it can result in double subscriptions being created at Stripe. This is due to setup_intent being reusable as it has to be to be used for recurring payments so Stripe doesn't reject the duplicate subscription. Implement safe guards against this by trying to find an existing subscription before making one to ensure the intent has not be used yet. That chances of this happening are quite slim though as it's subject to user error.