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/splash-page.html

@@ -6,7 +6,7 @@
 
 {% block platform_admin_content %}
 
-  <h1 class="heading-large">
+  <h1 class="heading-medium">
     Summary
   </h1>