Config: Add 'isProduction' flag

Check our env to determine whether we're
doing it live.
Use diff Stripe keys depending on isProduction()

Use the test api key if we're not in production,
and the live api key if we are.
Stripe: Absorb all webhook calls

We don't really care about reacting to the
Stripe webhook at this point, but we've
configured our Stripe app to send them
to us, and 404 errors are bothersome.
Refactor: Move Stripe public keys into config

... and only initialize the Stripe library on
the contribution page.
Fix: Config: Parse comma-separated lists

... account for undefined variables.