Feature/guest checkout

[i-just]

Description

This PR addresses a few bugs and slightly changes the behaviour of the src\services\Checkout->getCheckoutUrl() method to allow truly anonymous checkout.

• Fixed a bug where it wasn’t possible to use the checkout controller unless you were logged in to Craft
• Fixed a bug where we were attempting to set customer_email to null, causing a Stripe API error
• Fixed a few bugs in the README doc
• The src\services\Checkout->getCheckoutUrl() now accepts false as the $user parameter too. If null is passed (or the parameter is not explicitly used, in which case it’ll default to null), the method will continue to attempt to get the currently logged-in user. If false is passed, no such attempt will be made, the checkout URL will be anonymous, and the customer will have to fill out their email address on Stripe’s checkout page. (README adjusted accordingly.)
• The src\elements\Price->getCheckoutUrl() now accepts parameters ($customer, $successUrl, $cancelUrl, $params), all defaulting to null, and no longer assumes that the current user should always be used when generating the checkout URL.

Related issues

n/a

[i-just]

As per our discussions, I went back to accepting false and leaving the null behaviour as it was when generating checkout URLs.

This means that if you don’t pass anything as a $customer parameter, the default null value will be used, and the method will continue to attempt to get the currently logged-in user. However, if you pass false, the checkout URL will be anonymous (even if a user is currently logged in).

I also unified how the checkout is generated in all 3 cases: via controller action (stripe/checkout), twig function (stripeCheckoutUrl()) and price element method (price.getCheckoutUrl()).