[bug 1026956] Extend polls with no majority decision for 24 hours.

* Send reminder email marked as urgent on voting extension.
* Migrate remaining voting cronjobs to celery.
* Remove unused daily voting reminder cron.
Maybe this is a bit confusing, because it feels like a poll that is already extended. Something like `is_overdue` maybe will give the correct context. Feel free to choose something else, I am out of ideas :P