Show a tour when users first create a service

This commit adds a 3 screen tour, similar to those used on GOV.UK Verify
and Passports.

We guerilla tested this on Friday, and it really helped users to build a
mental model of how Notify works, so that when they’re playing around
with it they have a greater sense of what they’re aiming to do. This
makes concepts like templates and placeholders click more quickly.

https://www.pivotaltracker.com/story/show/116710119

tests/app/main/views/test_tour.py

@@ -0,0 +1,19 @@
+import pytest
+from flask import url_for
+
+import app
+
+
+@pytest.mark.parametrize("page", range(1, 5))
+def test_should_render_tour_pages(
+    app_,
+    api_user_active,
+    mocker,
+    page
+):
+    with app_.test_request_context():
+        with app_.test_client() as client:
+            client.login(api_user_active, mocker)
+            response = client.get(url_for('main.tour', service_id=101, page=page))
+            assert response.status_code == 200
+            assert 'Next' in response.get_data(as_text=True)