Disable stripe when running locally and a stripe api key is not provided

[oohwooh]

Right now, when running in development, and a STRIPE_PUBLISHABLE_KEY env is not provided, the site throws a (dismissable) error. To make developing www-event more accessible, we should suppress this error (perhaps by replacing the stripe PaymentProvider with some form of placeholder? This should log a warning in development, and throw a fatal error in production.