Add a Stripe webhook endpoint for payment processing

[exarkun]

This adds a handler at /v1/stripe/webhook which can be configured as a Stripe "webhook" for receiving push notifications about events related to a Stripe account. It is intended to be configured as a hook to receive checkout.session.completed events which it uses to populate the database with information about voucher payments.

A new required command-line option is introduced which must be given the "webhook secret" so that webhook request signatures can be verified to avoid use of the endpoint by parties other than Stripe.

The webhook must be configured in the correct Stripe account so that Stripe will make use of this endpoint. One way to do this is with:

curl \
    https://api.stripe.com/v1/webhook_endpoints \
    -u sk_test_yourkey: \
    -d url="https://serveraddress/v1/stripe/webhook" \
    -d "enabled_events[]"="checkout.session.completed"

The old "charge"-based endpoint is still in place but presumably it can be deleted in the near future (it seems convenient to have at least one revision where both systems are supported though).

Note that while the diff looks like it's a bit on the long side, a solid +400 of that diff are just sample Stripe JSON data.

Also note that this PR switches us to a fork of stripe-core which supports a newer version of the Stripe API. The changes in the fork are not currently suitable to be submitted upstream. Before we can satisfy further Stripe-related requirements we probably need to revisit our dependency on stripe-core (see #131 for one idea).

[hacklschorsch]

Reviewed to the best of my current ability, I feel this is a big improvement and should be merged & deployed (to be improved further in later change sets).

Added a few questions about semantics - where it looked to me naming didn't fit perfect to what the code does.

[hacklschorsch]

This includes / obsoletes #121.

[hacklschorsch]

I like the description of this MR a lot. At least the upper half of it would be a great permanent addition the the PaymentServer documentation IMHO

[exarkun]

I'm happy enough with this now. Assuming testing on staging goes well I'll merge it without further changes.

[exarkun]

This works on staging.