Reduce usage of the platform admin index page

This page is slow to load which means:
- it’s annoying for us
- it’s potentially causing load on the database

This commit does two things to reduce the amount we’re unnecessarily
looking at this page:

1. Avoid redirecting to it when signing in as a platform admin user
2. Don’t go directly to it when clicking ‘platform admin’ at the top,
   but instead show a holding page (there’s a fair chance you’ve clicked
   that link in order to go and manage some email branding or find a
   user, not wait for stats to load)

app/templates/admin_template.html

@@ -55,7 +55,7 @@
           "active": header_navigation.is_selected('user-profile')
         },
         {
-          "href": url_for('main.platform_admin'),
+          "href": url_for('main.platform_admin_splash_page'),
           "text": "Platform admin",
           "active": header_navigation.is_selected('platform-admin')
         },