Introduce `resource.allow_submission` for provisionally preventing job submissions

[tcompa]

This would be a boolean flag, always set to True upon resource creation.
Setting it to False would be useful so that we can leave fractal-server instance up even when the cluster should not be used (and then users should not try to submit jobs).

[jluethi]

Nice idea! :)

Would also be interesting to e.g. set this the evening or night before maintenance => if a job finishes before the cluster goes into maintenance, the next part of the workflow wouldn't be submitted anymore => fail gracefully