fundraising: 'customer.subscription.deleted' webhook event always gets 404 response

The response to stripe 'customer.subscription.deleted' webhook event is always 404. The reason is:

In the `cancel_donation` view, `stripe_subscription_id` is set to an empty string https://github.com/django/djangoproject.com/blob/e5a4bf20fb58c463441c15d552c022b03f716858/fundraising/views.py#L178

In the `subscription_cancelled` webhook handler method, the donation object is fetched using `stripe_subscription_id` which was set to an empty string earlier.

https://github.com/django/djangoproject.com/blob/e5a4bf20fb58c463441c15d552c022b03f716858/fundraising/views.py#L232-L233

So, the response will always be 404 and no cancellation email will be sent to the donor.

Start in fundraising/views.py at cancel_donation around line 178 and subscription_cancelled around lines 232–233. Trace the subscription ID between these handlers, then verify that a customer.subscription.deleted webhook can find the donation, returns successfully instead of 404, and sends the cancellation email.

Written by the indexing model from the issue text.