Add PayPal Checkout using PayPal REST API.

[argiepiano]

This has been prompted by the deprecation of PayPal WPS, WPP and EC. See this issue.

This feature should use Backdrop's commerce_paypal_checkout as a model. That module includes a light weight PHP library to authenticate, and uses PayPal Smart Button SDK to inject the checkout buttons in the last step.

I'm providing a first PR here. While basic checkout works, the PR does not yet work if you have UC Shipping enabled. I haven't yet tested with taxes either. This PR differs in some substantial ways from commerce_paypal_checkout since Ubercart does not allow for the addition of checkout pages. Therefore, this PR injects the buttons in the Review stage, and hides the form "Submit order" button, since the submission happens when the customer uses a smart button to complete the payment.

To test, create an App in your PayPal developer sandbox, and make a note of the Client ID and Secret. Then go to Ubercart's Store > Configuration > Payment methods, enable PayPal Checkout and enter the Client ID and secret.

At this point, I have only tested "Capture" transaction types, no "Authorize". Also the PayPal address collection is not yet enabled.

Please test. You can get some testing fake cc numbers here.

[argiepiano]

When testing, use settings to disable "Update billing customer profiles with address information the customer enters at PayPal". I haven't yet tested that.

[argiepiano]

The module is now able to pass shipping ("delivery") address and name, as well as shipping costs to PayPal, when uc_shipping is enabled. To make this work be sure to configure the module and deselect "Do not ask for a shipping address at PayPal."

[argiepiano]

The module now handles coupons (uc_coupon), discounts (uc_discounts), taxes (provided in U's core) and fees (uc_fee, not yet ported).

[argiepiano]

The module now handles authorizations correctly. I also added the ability to capture authorized payments, void authorizations and issue refunds for captured payments. All of this is done in new Actions added to the Payments tab.

TODO: add any Rules triggers as needed. Perhaps the same as the regular PayPal submodule?

[argiepiano]

I've added the Rules events for transactions: completed, pending, failed, voided and refunded.

[argiepiano]

I feel this is in pretty good shape to be released. We just need someone to "kick the tires".

[bugfolder]

I will, I will, just need to shake loose a big enough chunk of time during travels. For sure by mid-March, but no promises before then.