Refactor Woo checkout form validation

[crftwrk]

.woocommerce-invalid .form-control {
  @extend .is-invalid;
}

.woocommerce-validated .form-control {
  @extend .is-valid;
}

Much more better than

// Checkout Form Validation
$('body').on('blur change', '.form-row input', function () {
  $('.woocommerce form .form-row.woocommerce-validated .select2-container, .woocommerce form .form-row.woocommerce-validated input.input-text, .woocommerce form .form-row.woocommerce-validated select, .woocommerce form .form-row.woocommerce-validated .form-check-input[type=checkbox]').removeClass('is-invalid').addClass('is-valid');
  $('.woocommerce form .form-row.woocommerce-invalid .select2-container, .woocommerce form .form-row.woocommerce-invalid input.input-text, .woocommerce form .form-row.woocommerce-invalid select, .woocommerce form .form-row.woocommerce-invalid .form-check-input[type=checkbox]').removeClass('is-valid').addClass('is-invalid');
});

We can do this after #329 is merged