Scheduling strategy

[creepone]

For the parts of the workflow that are supposed to run in the background, we need some way to schedule them. Heroku uses a notion of background dynos for this, which are simple to use but paid (and relatively expensive, at least for this phase). We could use their syntax or any other way, e.g. having some cron jobs starting node with a worker script. I would rather do the scheduling at OS level than trying to mess with setTimeout-s in Javascript. The workers only have to be careful about not stepping on each other's toes, e.g. when processing emails, sending notifications etc.