Allow change of the PayPal button flavour

[erfanimani]

If your locale is not set to en_US, the PayPal module uses the really old button style:

 

It would be better for an admin to be able to choose the PayPal button flavour inside the payment method, and have the following choices:

https://developer.paypal.com/docs/classic/api/buttons/

Also, see Magento\Paypal\Model\Config line 983 (Magento 2.1.3)

[erfanimani]

For others stumbling across this idea - it's actually surprisingly easy to change. Create a plugin with `type` `name` `Magento\Paypal\Model\Config` with the following method:

    public function afterGetExpressCheckoutInContextImageUrl($localeCode, $result)
    {
        return "https://www.paypalobjects.com/webstatic/en_US/i/btn/png/silver-rect-paypalcheckout-34px.png";
    }