Make alert page work for rejected broadcasts

The code for this page was making assumptions about properties which
aren’t present on rejected broadcasts.

This commit accounts for those properties and presents the relevant
elements on the page.

tests/app/main/views/test_broadcast.py

@@ -1404,6 +1404,13 @@ def test_start_broadcasting(
         'Prepared by Alice and approved by Bob.',
         'Stopped by Carol yesterday at 9:21pm.',
     ]),
+    ('.view_rejected_broadcast', False, {
+        'status': 'rejected',
+        'updated_at': '2020-02-21T21:21:21.000000',
+    }, [
+        'Rejected yesterday at 9:21pm.',
+        'Prepared by Alice and approved by Bob.',
+    ]),
 ))
 @freeze_time('2020-02-22T22:22:22.000000')
 def test_view_broadcast_message_page(
@@ -1505,6 +1512,7 @@ def test_view_broadcast_message_shows_correct_highlighted_navigation(
             starts_at='2020-02-20T20:20:20.000000',
             finishes_at='2021-12-21T21:21:21.000000',
             cancelled_at='2021-01-01T01:01:01.000000',
+            updated_at='2021-01-01T01:01:01.000000',
             status=status,
         ),
     )