Implement a background job scheduler
To support "background thread" #47 (closed) and #48 (closed) we should implement a way to schedule execution of jobs/tasks in background. This can be used to schedule something "now", "every x minutes" or "every day". Those jobs/tasks should be visible to the users when applicable to the user and all tasks should be visible to the admins. We already have something very similar with the "Monitor" but it's not collaborative and everything is in memory. We should put something in database and have runner picking up tasks. Each instance of rdiffweb way have one or multiple runner. Those runner should execute the jobs/tasks ideally in a separate process.