Move PayPal client ID/token into ENV

Removed YAML configuration, moved into initializer. Here, we’re using a
default client ID and client secret provided by PayPal for sandbox use.
When running in prod, and dealing with live data from PayPal, we will
instead load the principal/credential from environment variables
instead. This gives us greater control over who is able to make calls
to PayPal on our behalf, and makes it easier to keep this credential
pair secret.