send users back to beginning of tour if they hit back at end

(previously it would have sent them to the choose template page)
if the user has added new templates or deleted the example one,
they're clearly competent enough to use the app so don't worry
(we wouldn't know what URL the tour starts on since the UUID of
the example template is random)

app/main/views/send.py

@@ -241,7 +241,7 @@ def _check_messages(service_id, template_type, upload_id, letters_as_pdf=False):
         # NOTE: this is a 301 MOVED PERMANENTLY (httpstatus.es/301), so the browser will cache this redirect, and it'll
         # *always* happen for that browser. _check_messages is only used by endpoints that contain `upload_id`, which
         # is a one-time-use id (that ties to a given file in S3 that is already deleted if it's not in the session)
-        raise RequestRedirect(url_for('main.choose_template', service_id=service_id, template_type=template_type))
+        raise RequestRedirect(get_check_messages_back_url(service_id, template_type))
 
     users = user_api_client.get_users_for_service(service_id=service_id)
 
@@ -410,3 +410,16 @@ def go_to_dashboard_after_tour(service_id, example_template_id):
     return redirect(
         url_for('main.service_dashboard', service_id=service_id)
     )
+
+
+def get_check_messages_back_url(service_id, template_type):
+    if get_help_argument():
+        # if the user is on the introductory tour, then they should be redirected back to the beginning of the tour -
+        # but to do that we need to find the template_id of the example template. That template *should* be the only
+        # template for that service, but it's possible they've opened another tab and deleted it for example. In that
+        # case we should just redirect back to the main page as they clearly know what they're doing.
+        templates = service_api_client.get_service_templates(service_id)['data']
+        if len(templates) == 1:
+            return url_for('.send_test', service_id=service_id, template_id=templates[0]['id'], help=1)
+
+    return url_for('main.choose_template', service_id=service_id, template_type=template_type)