Add platform–admin-only page to preview DVLA code

Debugging the DVLA markup stuff is quite slow, because it’s split
between the API and utils, and things that work in tests don’t always
work with real template content.

This commit adds us a private endpoint that will let us see what DVLA
markup will be generated for any template, enabling a quicker feedback
loop.

tests/app/main/views/test_templates.py

@@ -14,6 +14,9 @@ from tests.conftest import (
     mock_get_service_template,
     normalize_spaces,
     SERVICE_ONE_ID,
+    active_user_with_permissions,
+    platform_admin_user,
+    mock_get_user,
 )
 from tests import validate_route_permission, template_json, single_notification_json
 
@@ -1163,3 +1166,48 @@ def test_should_show_hint_once_template_redacted(
     )
 
     assert page.select('.hint')[0].text == 'Personalisation is hidden after sending'
+
+
+@pytest.mark.parametrize('test_user, template_mock, expected_response_code', [
+    (
+        active_user_with_permissions,
+        mock_get_service_letter_template,
+        403
+    ),
+    (
+        platform_admin_user,
+        mock_get_service_template,
+        404
+    ),
+    (
+        platform_admin_user,
+        mock_get_service_email_template,
+        404
+    ),
+    (
+        platform_admin_user,
+        mock_get_service_letter_template,
+        200
+    ),
+])
+def test_should_show_letter_template_as_dvla_markup(
+    client,
+    mocker,
+    service_one,
+    fake_uuid,
+    mock_get_service,
+    test_user,
+    template_mock,
+    expected_response_code,
+):
+
+    client.login(test_user(fake_uuid), mocker, service_one)
+    template_mock(mocker)
+
+    response = client.get(url_for(
+        '.view_template_as_dvla_markup',
+        service_id=SERVICE_ONE_ID,
+        template_id=fake_uuid,
+    ))
+
+    assert response.status_code == expected_response_code