Validate checkout form before sending request to PayPal

[strangerkir]

Issue: PCP-75.

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Description

Changes the create order process so the checkout form validated before sending create order request to PayPal. This prevents cases of sending incomplete data to PayPal and getting errors from there.

Steps to test:

1. Add any product to the cart.
2. Go to the checkout page.
3. Select PayPal as a payment method.
4. Fill the checkout form except for one required field (e.g. City/Town).
5. Submit the checkout.

You should see the standard WooCommerce error like "Town / City is a required field." instead of an error like
"[UNPROCESSABLE_ENTITY] The requested action could not be performed..." that is returned from PayPal.

Documentation

• This PR needs documentation (has the "Documentation" label).

Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

• Fix - Checkout form validation.

Closes #115.