Create integration tests that use the Stripe webhook handler to configure a plan

[travis]

It would be nice to have a test for our Stripe integration. I think this can be added to the existing integration test flow in test/integration.test.js, but we will need to coordinate webhook signing secrets and call into the billing service for the first time, so I think it will take a bit of work to do properly. The following is a rough outline of how I think this should work:

1. we will need to add an additional step to setupNewClient that uses stripe.webhooks.generateTestHeaderString to create a signed webhook request
2. we'll need to POST that signed request to the billing service's new /stripe endpoint - this will require us to teach setupNewClient about the billing service, since it uses a different HTTP endpoint than the upload API we've interacted with thus far
3. we'll need to ensure that the test environment's secrets are configured with a STRIPE_ENDPOINT_SECRET matching the secret used to create the signed webhook request in step (1) - this probably means adding a STRIPE_ENDPOINT_TEST_SECRET (name flexible) to the process.env of the integration test and ensuring that the two match. We'll want to be careful to avoid leaking secrets if possible here.
4. we'll need to ensure that the REQUIRE_PAYMENT_PLAN environment variable is set to true in the integration environment.