feat: integrate with MailJet API for newsletter registration

templates/booking/create/step_4.html.twig

@@ -546,6 +546,11 @@
                             Bestätigung
                         </div>
                         <div class="p-4">
+                            {% if form.newsletterOptIn is defined and newsletterOptInVisible and newsletterTargetEmail %}
+                                {{ form_row(form.newsletterOptIn, {
+                                    'label_attr': {'class': 'text-base font-semibold'}
+                                }) }}
+                            {% endif %}
                             {{ form_row(form.confirmationAccepted, {
                                 'label_attr': {'class': 'text-base font-semibold'}
                             }) }}