Tidy up the platform admin pages

This commit does two things:
- brings the ‘All organisations’ page back within the platform admin
  part of the site (because it’s hard to find otherwise)
- makes the layouts of all the pages within platform admin a bit closer
  to the service-specific pages in terms of heading sizes, spacing, etc
  so that moving between them doesn’t feel so jumpy

app/templates/views/platform-admin/clear-cache.html

@@ -4,13 +4,13 @@
 {% from "components/page-footer.html" import page_footer %}
 
 {% block per_page_title %}
-  Clear Cache
+  Clear cache
 {% endblock %}
 
 {% block platform_admin_content %}
 
-  <h1 class="heading-large">
-    Clear Cache
+  <h1 class="heading-medium">
+    Clear cache
   </h1>
 
   {% call form_wrapper() %}