Make it easy to see API keys for broadcast service

This made it easier to debug a problem with the functional tests
due to the fixtures not working correctly [^1]. It's a platform
admin only convenience over knowing the page URL.

We may want to expose the top-level "/api-integration" page but
that will require more work to show which broadcasts were sent with
which key - currently it's oriented around "messages". For now I
think it's useful to see what keys a service has.

[^1]: https://github.com/alphagov/notifications-functional-tests/pull/411#pullrequestreview-920069799

tests/app/test_navigation.py

@@ -547,6 +547,35 @@ def test_navigation_for_services_with_broadcast_permission(
     ]
 
 
+def test_navigation_for_services_with_broadcast_permission_platform_admin(
+    mocker,
+    client_request,
+    service_one,
+    mock_get_service_templates,
+    mock_get_template_folders,
+    mock_get_api_keys,
+    platform_admin_user,
+):
+    service_one['permissions'] += ['broadcast']
+    mocker.patch(
+        'app.user_api_client.get_user',
+        return_value=platform_admin_user,
+    )
+
+    page = client_request.get('main.choose_template', service_id=SERVICE_ONE_ID)
+    assert [
+        a['href'] for a in page.select('.navigation a')
+    ] == [
+        '/services/{}/current-alerts'.format(SERVICE_ONE_ID),
+        '/services/{}/past-alerts'.format(SERVICE_ONE_ID),
+        '/services/{}/rejected-alerts'.format(SERVICE_ONE_ID),
+        '/services/{}/templates'.format(SERVICE_ONE_ID),
+        '/services/{}/users'.format(SERVICE_ONE_ID),
+        '/services/{}/service-settings'.format(SERVICE_ONE_ID),
+        '/services/{}/api/keys'.format(SERVICE_ONE_ID),
+    ]
+
+
 def test_caseworkers_get_caseworking_navigation(
     client_request,
     mocker,