[13.x] Server side Checkout redirect

[driesvints]

This PR implements some nice API's to Stripe's recent addition of exposing the Checkout url on its session object. This will greatly simplify the workflow for a library like Cashier to use redirects instead of doing it through Stripe.js

Before:

$checkoutBtn = Auth::user()->checkout('price_tshirt')->button();

return view('checkout', compact('checkoutBtn'));

And have the customer click a button. Now:

return Auth::user()->checkout('price_tshirt');

I've deprecated the button method because I think that from now on that's going to be a very rare use case.

[driesvints]

I chose 303 on purpose here because Stripe recommends it in their docs (and @cjavilla-stripe did in one of his video tutorials): https://stripe.com/docs/payments/accept-a-payment?platform=web&ui=checkout#redirect-customers