Support background jobs

[JeanMertz]

At Blendle we have a use-case for starting a job that can take up to 15 minutes.

Right now, the job runs in a background job processor (Sidekiq), so the response you get from starting a task is that "the job has started, if you do not see the expected outcome in 15 minutes, ask ...".

This is obviously not great, and causes confusion because, well... people don't always read the response, they simply saw a green response and figured it's done.

Even if they did read the response, the result is that they have to wait 15 minutes, then go check the thing they started to see if it works, which might not always be easy or obvious, and then have to make a decision on whether it worked or not, and whether or not to ask someone to help them out.

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Ideally, Automaat would be able to run these job asynchronously on the server. The clients could then implement some view that allows you to see running jobs of a task, and you'd get a notification of some kind if a job completed.

This is related to #34 and #18.

Some thoughts:

• This would require some UI changes to be able to see running jobs in the task details view
• Currently the web-client polls the server every x seconds to check for results, partly because the GraphQL library used does not support subscriptions, and partially because it was the easiest to implement. For now this can stay, but I guess it would become a bit different, polling the server instead for "are there any jobs I'm interested in still running? or did one of them succeed?"