Management command without cron

[ligi]

currently we run management commands every minute via cron to confirm orders or deal with refunds. But when people would install via the marketplace after #158 - then this is suboptimal. It would be better if it could run in pretix context triggered/scheduled by the plugin.
Maybe we can hook into here: https://docs.pretix.eu/en/latest/development/api/general.html#pretix.base.signals.periodic_task

[kvbik]

this is being worked on here rahul-kothari#9