plugin: enforce `preemptible-after` limits, set `preemptible-after` default

[cmoussa1]

The priority plugin will need to do two separate things concerning preemptible-after attributes for a job:

1. If an association does not set a preemptible-after attribute for their job and they are running under a special bank that has max-preemptible-after configured, the job should be updated to include this attribute. This could probably happen in the callback for job.new, where default bank and default project name jobspec-updates are also currently happening.
2. If an association does set a preemptible-after attribute for their job, it should be validated to make sure that the value is not greater than the max-preemptible-after value set for the bank. If it is, it should automatically be lowered to whatever the max is. I'm thinking this could probably also happen in the callback for job.new since jobs won't be rejected in case the value is too high, it'll just be updated.