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/service-settings/sms-sender/edit.html

@@ -3,17 +3,21 @@
 {% from "components/page-header.html" import page_header %}
 {% from "components/page-footer.html" import page_footer %}
 {% from "components/form.html" import form_wrapper %}
+{% from "components/back-link/macro.njk" import govukBackLink %}
 
 {% block service_page_title %}
   Change text message sender
 {% endblock %}
 
+{% block backLink %}
+  {% if not request.args.get('from_template') %}
+    {{ govukBackLink({ "href": url_for('.service_sms_senders', service_id=current_service.id) }) }}
+  {% endif %}
+{% endblock %}
+
 {% block maincolumn_content %}
 
-  {{ page_header(
-    'Change text message sender',
-    back_link=None if request.args.get('from_template') else url_for('.service_sms_senders', service_id=current_service.id)
-  ) }}
+  {{ page_header('Change text message sender') }}
   {% call form_wrapper() %}
     {% if inbound_number %}
       <p class="govuk-body">