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/two_factor.py

@@ -111,7 +111,5 @@ def redirect_when_logged_in(platform_admin):
     next_url = request.args.get('next')
     if next_url and _is_safe_redirect_url(next_url):
         return redirect(next_url)
-    if platform_admin:
-        return redirect(url_for('main.platform_admin'))
 
     return redirect(url_for('main.show_accounts_or_dashboard'))