Reduce default broadcast expiry time to 4 hours

We don’t think we need to broadcast longer than this to validate that
the system is working.

app/models/broadcast_message.py

@@ -220,7 +220,7 @@ class BroadcastMessage(JSONModel):
         self._update(
             starts_at=datetime.utcnow().isoformat(),
             finishes_at=(
-                datetime.utcnow() + timedelta(hours=23, minutes=59)
+                datetime.utcnow() + timedelta(hours=4, minutes=0)
             ).isoformat(),
         )
         self._set_status_to('broadcasting')