Add CSP: style-src -> assets.braintreegateway.com (#746)
Add CSP: style-src -> assets.braintreegateway.com (#746)

Without this, we get such error:

> [Error] Refused to load https://assets.braintreegateway.com/web/dropin/1.30.1/css/dropin.min.css because it does not appear in the style-src directive of the Content Security Policy.
braintree/braintree-web-drop-in,braintree/braintree-web-drop-in,braintree/braintree-web-drop-in