Implement queuing priority #56

[patrick-austin]

Also includes changes from #45 #51, due to dependency of priority settings on submitting to and polling the queue. See those PRs for those changes in isolation.

• Add queue priority settings to run.properties.
  • This can be a single number, for all users, authenticated users, investigation users or instrument scientists.
  • For the latter two, it can also be a mapping of Instrument.name or role to a priority level
• Class to handle loading and formatting priority levels in JPQL
• Added functions to IcatClient to perform these queries for a given user to identify their level of priority
• Modified StatusCheck to get all queued Downloads, then when iterating only submit those with priority 1. If there is still space, start submitting from priority 2 and so on.
• For queue submission endpoints, now only submit if user has a positive priority

Closes #56