dont use has_permissions to check for platform admins

platform_admin is a separate concept to permissions, so by removing the
checks for it from the current_user.has_permissions function, we can
simplify things greatly. We already record on the user whether they're
a platform admin anyway.

app/templates/views/service-settings.html

@@ -250,7 +250,7 @@
 
     {% endif %}
 
-    {% if current_user.has_permissions(admin_override=True) %}
+    {% if current_user.platform_admin %}
 
       <h2 class="heading-medium">Platform admin settings</h2>
       {% call mapping_table(