Add a tour for users new to broadcast services

This is an initial, prototype-quality attempt at introducing some kind
of tour for users new to broadcasting. A lot of the users we’re speaking
to don’t have a good concept of what broadcasting means, which is
causing usability problems down the line.

We did a similar thing in the early days of Notify to explain the
concept of message templates and personalisation.

tests/app/main/views/test_broadcast.py

@@ -94,6 +94,7 @@ def test_empty_broadcast_dashboard(
     client_request,
     service_one,
     mock_get_no_broadcast_messages,
+    mock_get_service_templates_when_no_templates_exist,
 ):
     service_one['permissions'] += ['broadcast']
     page = client_request.get(
@@ -114,6 +115,7 @@ def test_broadcast_dashboard(
     client_request,
     service_one,
     mock_get_broadcast_messages,
+    mock_get_service_templates,
 ):
     service_one['permissions'] += ['broadcast']
     page = client_request.get(