refs https://github.com/TryGhost/Team/issues/1338
This adds some initial scaffolding to make it easier to test Members with Stripe
- `mockStripe` method to disable the network, so we can use nock to intercept
Stripe API calls
- `setupStripe` method to be called _before_ getting an agent so that Stripe is
configured
- `getMembersAPIAgent` to get an agent for the Members frontend API
