UI: Allow jobs to be reset to pending

[smathot]

Once jobs are set to "finished," there's no way to set them back to "pending." Add a button for this.

[BmBaczkowski]

Could you provide more details on how this should be implemented? In d0734d2 I implemented reseting specific jobs per participant per study by adding "refresh" icons in Study -> Participants; next to setting priority. It opens a dialog box to select which (or all) jobs should be rest - available only when there are finished jobs.

[smathot]

I didn't have a specific implementation in mind, but this seems very reasonable and provides all the functionality needed as far as I can tell. 👍

[BmBaczkowski]

There is a bug. Potentially similar problem to #226