There are (at least) two types of objects that could be sent with a
charge.succeeded event, a Charge (e.g. for credit cards) or a Payment (if
they pay by ACH). We were handling the first but not the second.
This commit also updates the fixture for the existing charge.succeeded event
to the latest API version.
Recent changes merged in #10877 didn't handle these events
correctly. The linkified_id function breaks for the `discount`
object in the JSON payload. A cursory glance at Stripe's docs
tells me that since a discount is associated with a customer
or a coupon, it makes sense for a `discount` object to not have
an ID that can necessarily be linked to. So, we can just link
to the associated coupon instead.
All webhook fixtures in zerver/fixtures/<webhook_name> have now
been moved to dedicated webhook-specific directories under
zerver/webhooks/<webhook_name>/fixtures, where <webhook_name> is
the name of the webhook.