Add backLink block to templates

This will be used to put the back link in, since it it is before the
`<main>` tag. We could have used the `beforeContent` block directly, but
that sometimes already has content in - this means it's not clear when
you also need to use `super()` inside the block and when you don't.

app/templates/views/platform-admin/_base_template.html

@@ -9,6 +9,7 @@
     </div>
     <a href="{{ url_for('main.choose_account') }}" class="govuk-link govuk-link--no-visited-state navigation-service-switch">Switch service</a>
   </div>
+  {% block backLink %}{% endblock %}
 {% endblock %}
 
 {% block content %}