VidGrind cron mode

Issue #275 resolved
Alan Noble created an issue

It is highly desirable to be to run multiple instances of instances of VidGrind, while still limiting cron scheduling to a single instance. This is readily achieved by deploying VidGrind in cron mode (ENABLE_CRON=true) as a separate web service, which is limited to one instance. Conversely, when deployed normally, VidGrind can scale to as many instances as required.

As a bonus, it will only be necessary to re-deploy the cron service when cron functionality changes and the cron scheduler will not need to be restarted as office.