webhooks: Handle dispute events with object IDs prefixed with du.
webhooks: Handle dispute events with object IDs prefixed with du.

Sometimes the dispute object IDs are prefixed with `du` instead of `dp`.

https://freenode.logbot.info/stripe/20200605#c4059469

The correct long-term fix here would be to stop using object IDs to
detect the object type of these events and instead maybe make use of
"object" key instead.

https://stripe.com/docs/api/disputes/object#dispute_object-object

punchagan/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,showell/zulip,showell/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,showell/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,rht/zulip,punchagan/zulip,andersk/zulip,punchagan/zulip,andersk/zulip,kou/zulip,showell/zulip,zulip/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,eeshangarg/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,showell/zulip,kou/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,kou/zulip,andersk/zulip,hackerkid/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,rht/zulip,showell/zulip,hackerkid/zulip,showell/zulip,andersk/zulip