Fix pages which assume broadcasts have a finish time

They no longer have a finish time until they have been approved.
Previously it was the person preparing the broadcast who chose when it
should finish.

tests/app/main/views/test_broadcast.py

@@ -760,7 +760,6 @@ def test_cant_approve_own_broadcast(
         normalize_spaces(page.select_one('.banner').text)
     ) == (
         'Your broadcast is waiting for approval from another member of your team '
-        'Once approved it will be live until tomorrow at 11:23pm '
         'Withdraw this broadcast'
     )
 

tests/conftest.py

@@ -4284,6 +4284,7 @@ def mock_get_broadcast_messages(
             ),
             partial_json(
                 status='pending-approval',
+                finishes_at=None,
             ),
             partial_json(
                 status='broadcasting',