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)

tests/app/main/views/test_platform_admin.py

@@ -37,6 +37,7 @@ def test_should_redirect_if_not_logged_in(
 
 @pytest.mark.parametrize('endpoint', [
     'main.platform_admin',
+    'main.platform_admin_splash_page',
     'main.live_services',
     'main.trial_services',
 ])
@@ -589,6 +590,20 @@ def test_get_tech_failure_status_box_data_removes_percentage_data():
     assert 'percentage' not in tech_failure_data
 
 
+def test_platform_admin_splash_doesnt_talk_to_api(
+    client_request,
+    platform_admin_user,
+):
+
+    client_request.login(platform_admin_user)
+
+    page = client_request.get('main.platform_admin_splash_page')
+
+    assert page.select_one('main .govuk-body a')['href'] == url_for(
+        'main.platform_admin',
+    )
+
+
 def test_platform_admin_with_start_and_end_dates_provided(mocker, platform_admin_client):
     start_date = '2018-01-01'
     end_date = '2018-06-01'