Add a way to configure SLURM `#SBATCH --output`

[t20100]

Proposed change

It would be nice to allow the configuration of the --output sbatch parameter the same way as for other parameters.
This would enable customisation of the output file as well as passing /dev/null to silent SLURM output.

Alternative options

Leave it as it is: --output={{homedir}}/jupyterhub_slurmspawner_%j.log

Who would use this feature?

I'm already using a similar feature in jupyterhub_moss, a customized SLURM Spawner (See https://github.com/silx-kit/jupyterhub_moss/blob/fbcf07c7b7198af97e8cd3545458759e5e882b9e/jupyterhub_moss/batch_script.sh#L13), but this requires duplicating the batch_script.

(Optional): Suggest a solution

Add a req_output to SlurmSpawner (see #258).

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