Show rejected broadcasts on ‘Previous alerts’ page

Two reasons to not hide rejected broadcasts:
- if a broadcast was rejected by mistake then it’s useful to have an
  audit of who did that
- it means you can still see old broadcasts without having to leave
  in pending-approval, which is dangerous because they might
  accidentally be approved

tests/app/main/views/test_broadcast.py

@@ -420,6 +420,7 @@ def test_previous_broadcasts_page(
         normalize_spaces(row.text)
         for row in page.select('.ajax-block-container')[0].select('.file-list')
     ] == [
+        'Example template This is a test Rejected today at 1:20am England Scotland',
         'Example template This is a test Broadcast yesterday at 2:20pm England Scotland',
         'Example template This is a test Broadcast yesterday at 2:20am England Scotland',
     ]