Fix invalid client_reference_id composition

[ledermann]

I've been trying to integrate the Stripe pricing table without success. I realized that the current method Pay::Stripe.to_client_reference_id is creating an invalid string. It includes a forward slash (like User/42), which isn't allowed according to Stripe's docs:

The web component supports setting the client-reference-id property. When the property is set, the pricing table passes it to the Checkout Session’s client_reference_id attribute to help you reconcile the Checkout Session with your internal system. This can be an authenticated user ID or a similar string. client-reference-id can be composed of alphanumeric characters, dashes, or underscores, and be any value up to 200 characters. Invalid values are silently dropped and your pricing table will continue to work as expected.

Source: https://stripe.com/docs/payments/checkout/pricing-table#handle-fulfillment-with-the-stripe-api

The resulting webhook checkout.session.completed then contains client_reference_id: null, so the client cannot be found.

I changed the implementation to use an underscore instead of the slash (like User_42).

References: #670, #823