Cron scheduler control

It is necessary to enable or disable a VidGrind instance from running the cron scheduler. For example, when deploying a test version of VidGrind, it is necessary to disable that instance’s cron scheduler in order to avoid multiple schedulers from running. An ENABLE_CRON env var is proposed, set to true or false.

This env var could also be used to ensure that one and only one VidGrind instance ever runs the cron scheduler.