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.

app/templates/views/broadcast/dashboard.html

@@ -10,6 +10,14 @@
 
   <h1 class="govuk-visually-hidden">Dashboard</h1>
 
+  {% if current_user.has_permissions('manage_templates') and not current_service.all_templates %}
+    <nav class="govuk-!-margin-top-2 govuk-!-margin-bottom-6">
+      <a class="govuk-link govuk-link--no-visited-state pill-separate-item govuk-!-padding-top-4 govuk-!-padding-bottom-4 govuk-!-font-weight-bold" href="{{ url_for('.choose_template', service_id=current_service.id) }}">
+        Start by creating a template
+      </a>
+    </nav>
+  {% endif %}
+
   <h2 class="heading-medium govuk-!-margin-bottom-2">Live broadcasts</h2>
 
   {{ ajax_block(