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/disable-platform-admin-view.html

@@ -9,11 +9,14 @@
   {{ page_title }}
 {% endblock %}
 
+{% block backLink %}
+  {{ govukBackLink({ "href": url_for('.user_profile') }) }}
+{% endblock %}
+
 {% block maincolumn_content %}
 
   <div class="govuk-grid-row">
     <div class="govuk-grid-column-five-sixths">
-      {{ govukBackLink({ "href": url_for('.user_profile') }) }}
 
       {% call form_wrapper(class="govuk-!-margin-top-3") %}
         {{ form.enabled(param_extensions={