Support on/off schedule for apps

[DouweM]

The cron schedule can already be modified to only update during certain hours, but the app will still show up on the Tidbyt -- just outdated. There's no Mute API, so we'd need to delete it when turning it off. That'd mean that when turning it on again, it'll show up at the end of all the apps instead of the place it had originally, which is not ideal. A Mute API would be great!