Create "visible by customer' checkbox to orders admin view

[tvdeyen]

Currently we are hard coding frontend_viewable: false into the order params in the admin orders controller. What about making this a checkbox on the admin orders screen? Then a customer agent could decide if the order created is visible by the customer or not.

Originally posted by @tvdeyen in https://github.com/solidusio/solidus/issues/6362#issuecomment-3489650281