Move back link outside of main where it was used in the page header

The page_header macro includes an optional back link. Since the
page_header is always used inside `<main>`, where the back link should
not be, this stops setting the back link in the page header and instead
sets it in the new `backLink` block.

app/templates/views/api/guest-list.html

@@ -5,11 +5,16 @@
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/list-entry.html" import list_entry %}
 {% from "components/form.html" import form_wrapper %}
+{% from "components/back-link/macro.njk" import govukBackLink %}
 
 {% block service_page_title %}
   Guest list
 {% endblock %}
 
+{% block backLink %}
+  {{ govukBackLink({ "href": url_for('main.api_integration', service_id=current_service.id) }) }}
+{% endblock %}
+
 {% block maincolumn_content %}
 
   {% if form.email_addresses.errors or form.phone_numbers.errors %}
@@ -32,10 +37,7 @@
       </ul>
     {% endcall %}
   {% else %}
-    {{ page_header(
-      'Guest list',
-      back_link=url_for('main.api_integration', service_id=current_service.id)
-    ) }}
+    {{ page_header('Guest list') }}
   {% endif %}
 
   <p class="govuk-body">