Look into whether a `job.numDesiredAllocs` property is feasible

For the updated job status panel, we'd like to show "X of Y Allocations Running", where Y is the desired/intended count.

For Service jobs, this can be determined by adding up group counts.

For System jobs, this is harder: we need to know the logic of the constraint scheduler, among other things.

Would it be possible to get a tally of the nodes the job SHOULD be running on as a property on said job?