Add page to choose services

Because a user can have multiple services, they need a way to navigate between
them. Normally they can use the ▶ Switcher to do this, except when:

- they first sign in
- they are on a page which isn’t associated with a service (eg user profile) in
  which case we can’t use the switcher because it won’t know what the ‘current’
  service is

So this commit adds a new page with a (fake) list of services.

tests/app/main/views/test_choose_services.py

@@ -0,0 +1,15 @@
+from tests.app.main import create_test_user
+from flask import url_for
+
+
+def test_should_show_choose_services_page(notifications_admin,
+                                          notifications_admin_db,
+                                          notify_db_session):
+    with notifications_admin.test_request_context():
+        with notifications_admin.test_client() as client:
+            user = create_test_user('active')
+            client.login(user)
+            response = client.get('/services')
+
+        assert response.status_code == 200
+        assert 'Choose service' in response.get_data(as_text=True)