Add a link to reject a broadcast

If a broadcast definitely shouldn’t go out (for example because it has a
spelling mistake or is going to the wrong areas) then we should have a
way of removing it. Once it’s removed no-one else can approve it, and it
isn’t cluttering up the dashboard.

This is a link (because it’s a secondary action) and red (because it’s
destructive, in that it’s throwing away someone’s work).

app/templates/views/broadcast/view-message.html

@@ -23,7 +23,9 @@
         message until {{ broadcast_message.finishes_at|format_datetime_relative }}.
       </p>
       {{ page_footer(
-        "Start broadcasting now"
+        "Start broadcasting now",
+        delete_link=url_for('main.reject_broadcast_message', service_id=current_service.id, broadcast_message_id=broadcast_message.id),
+        delete_link_text='Reject this broadcast'
       ) }}
     {% endcall %}
   {% else %}