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/main/views/platform_admin.py

@@ -42,6 +42,14 @@ ZERO_FAILURE_THRESHOLD = 0
 
 @main.route("/platform-admin")
 @user_is_platform_admin
+def platform_admin_splash_page():
+    return render_template(
+        'views/platform-admin/splash-page.html',
+    )
+
+
+@main.route("/platform-admin/summary")
+@user_is_platform_admin
 def platform_admin():
     form = DateFilterForm(request.args, meta={'csrf': False})
     api_args = {}