Add a setting to suppress the credentials message after Stripe Connect

Some sites using Stripe Connect on behalf of third party merchants, may prefer not to show any sort of credentials message upon a successful return from Stripe. Right now, their primary option would be to override a controller.

Can we add a condition to the messaging that lets us suppress that message, perhaps based on a variable set in settings.php like we've done elsewhere? Another option would be to just make a toggle in the global "Stripe settings" form to suppress that message and / or add a textarea for a custom message to be defined.