Improve pacing and sequence of information in the broadcast tour

This commit refines which information we show on each page.

Specifically we’re
- adding some wording (‘at exactly the same time’) to try to communicate
  the immediacy
- giving the ‘loud noises’ message it’s own screen to really draw
  attention to it
- moving the ‘no phone numbers bit’ later in the journey, and
  experimenting with explaining why that is, to make it clearer how it’s
  different to a text message

app/main/views/broadcast.py

@@ -23,7 +23,7 @@ from app.utils import service_has_permission, user_has_permissions
 @user_has_permissions()
 @service_has_permission('broadcast')
 def broadcast_tour(service_id, step_index):
-    if step_index not in (1, 2, 3, 4):
+    if step_index not in (1, 2, 3, 4, 5):
         abort(404)
     return render_template(
         f'views/broadcast/tour/{step_index}.html'