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/models/broadcast_message.py

@@ -144,6 +144,9 @@ class BroadcastMessage(JSONModel):
         )
         self._set_status_to('broadcasting')
 
+    def reject_broadcast(self):
+        self._set_status_to('rejected')
+
     def cancel_broadcast(self):
         self._set_status_to('cancelled')