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/views/platform-admin/splash-page.html

@@ -0,0 +1,17 @@
+{% extends "views/platform-admin/_base_template.html" %}
+
+{% block per_page_title %}
+  Summary
+{% endblock %}
+
+{% block platform_admin_content %}
+
+  <h1 class="heading-large">
+    Summary
+  </h1>
+
+  <p class="govuk-body">
+    <a class="govuk-link govuk-link--no-visited-state" href="{{ url_for('main.platform_admin') }}">Load summary</a>
+  </p>
+
+{% endblock %}