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/inbound-sms-admin.html

@@ -20,15 +20,9 @@ Inbound Numbers
 
 {% block platform_admin_content %}
 
-  <div class="govuk-grid-row bottom-gutter">
-    <div class="govuk-grid-column-two-thirds">
-      <h1 class="heading-large">
-        Inbound SMS
-      </h1>
-    </div>
-  </div>
-
-
+  <h1 class="heading-medium">
+    Inbound SMS
+  </h1>
 
   <table class="inbound">
           <col style="width:8%">