Add some styling to broadcast messages

This is borrowed from an earlier prototype, and is meant to be temporary
– something better than just plain text.

Text in generated content isn’t always announced by screen readers, so
we should definitely move away from that once we understand what text
will be shown on the phone and where it comes from.

app/templates/views/broadcast/preview-areas.html

@@ -58,7 +58,7 @@
 
 {% block maincolumn_content %}
 
-  {{ page_header("Choose where to broadcast to", back_link="{{ url_for('.broadcast', service_id=current_service.id) }}") }}
+  {{ page_header("Choose where to broadcast to", back_link=url_for('.choose_template', service_id=current_service.id)) }}
 
   {% for area in selected %}
     {% if loop.first %}