Allow broadcasts to be cancelled

Currently this is a `get` request from the dashboard. Once we have a page
for viewing an individual broadcast it should probably show there
instead and be a `get`/confirm/`post` loop like for deleting a template.

app/main/views/broadcast.py

@@ -124,3 +124,17 @@ def preview_broadcast_message(service_id, broadcast_message_id):
         'views/broadcast/preview-message.html',
         broadcast_message=broadcast_message,
     )
+
+
+@main.route('/services/<uuid:service_id>/broadcast/<uuid:broadcast_message_id>/cancel')
+@user_has_permissions('send_messages')
+@service_has_permission('broadcast')
+def cancel_broadcast_message(service_id, broadcast_message_id):
+    BroadcastMessage.from_id(
+        broadcast_message_id,
+        service_id=current_service.id,
+    ).cancel_broadcast()
+    return redirect(url_for(
+        '.broadcast_dashboard',
+        service_id=current_service.id,
+    ))