Replace whenever gem with a sidekiq plugin

[ysbaddaden]

The whenever gem doesn't fit so well into a Docker environment: it depends on the crontab, must be reconfigured on each application restart, and so on. I'm not even sure the crontab actually ever runs in the container (we only update the crontab, there is cron daemon service running), so I'm not sure the temporarily uploaded assay files are ever being cleaned up.

It would also simplify the Docker images: no need to install cron, to run whenever --update-crontab, and so on.

We have a sidekiq service, and there are a couple of maintained plugins to use it for recurring jobs in addition to one time jobs:

• sidekiq-scheduler
• sidekiq-cron

I used 'em both. I recall I liked one more than the other, but I don't remember which.

[ysbaddaden]

Oh, we already have sidekiq-cron. Let's use it 🎉