Have an option for disable-by-default for cron entries

[bensheldon]

I took down one of my apps because I had an invalid cron: definition that was only configured to run in my production environment (not staging, not test).

I'm thinking that there should be an option like default_disable: true or default_enabled: false that would run through all of the cron configuration, but would require explicitly going into the dashboard and enabling it in a particular environment (or GoodJob::CronEntry.find(key).enable).

I think someone suggested an idea like this already, but I couldn't find that.