Move back link above main content for straightforward cases

This moves the back link to be above the `<main>` tag by making use of
the new `backLink` block. This doesn't change the pages which are using
a back link as part of the `page_header` macro yet.

app/templates/views/user-profile/security-keys.html

@@ -17,6 +17,10 @@
   {{ page_title }}
 {% endblock %}
 
+{% block backLink %}
+  {{ govukBackLink({ "href": url_for('.user_profile') }) }}
+{% endblock %}
+
 {% block maincolumn_content %}
 
 {% set webauthn_button %}
@@ -41,8 +45,6 @@
   }) }}
 {% endset %}
 
-
-  {{ govukBackLink({ "href": url_for('.user_profile') }) }}
   <div class="govuk-grid-row">
     {% if credentials %}
       <div class="govuk-grid-column-five-sixths">