Verify that no webhooks accept data from the event directly

[joshsmith]

Problem

Per #612 we should ensure that webhooks work idempotently by simply fetching the latest data from Stripe's API.

We'll need to step through each endpoint to verify this is the case.

[begedin]

Went through all of our current webhooks handlers and none of them use the data stored in the event at the moment, so this should be safe to close.