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/navigation.py

@@ -98,6 +98,7 @@ class HeaderNavigation(Navigation):
             'notifications_sent_by_service',
             'performance_platform_xlsx',
             'usage_for_all_services',
+            'organisations',
             'platform_admin',
             'platform_admin_list_complaints',
             'platform_admin_reports',
@@ -235,7 +236,6 @@ class HeaderNavigation(Navigation):
         'organisation_settings',
         'organisation_preview_email_branding',
         'organisation_preview_letter_branding',
-        'organisations',
         'privacy',
         'public_agreement',
         'public_download_agreement',