Feature: add support to server validations for PayPal Donations gateway

[glaubersilva]

Resolves GIVE-2043

Description

This PR adds a request to validate the donation form on the server side when the donor clicks on the PayPal Donations smart buttons, this way we can prevent the creation of donations on the PayPal side if the form has some invalid data with custom validation rules that only can be triggered on the server side.

Also, it removes the work around implemented a while ago to validate the billing address block before creating the PayPal donation, this is not more necessary now that we have the validations being triggered on the server-side.

This change can potentially prevent any future problem related to validations not being triggered for the PayPal Donations gateway because now we can ensure that all validations on both sides (client and server) will always be triggered before the payment process.

Affects

The PayPal Donations gateway.

Visuals

Testing Instructions

1. Add the billing address block to a donation form;
2. Leave some required address fields empty;
3. Try to create a test donation using the PayPal Donations gateway;
4. You should have the donation blocked and see some error messages next to the empty fields.

Pre-review Checklist

• Acceptance criteria satisfied and marked in related issue
• Relevant @unreleased tags included in DocBlocks
• Includes unit tests
• Reviewed by the designer (if follows a design)
• Self Review of code and UX completed

[kjohnson]

Great find, @glaubersilva! Leveraging the validation endpoint used by the multi-step form is a good idea. Glad to see that we do not need to refactor shared PayPal code to resolve this issue.

There are a couple of non-blocking comments for you consideration.

[rickalday]

Passed QA tests