[ingvarch]

What changes

Volumes, CSI plugins and variables now update as soon as they change, the way
jobs and allocations do, and o on a job or an allocation opens :events for
that one object only.

• :volumes, a volume, :plugins follow the event stream on Nomad 1.10 and
  later; :variables and a variable on Nomad 2.0 and later. A cluster does not
  refuse a topic it does not know when ACLs are off, it just never sends
  anything, so urga checks the version of the cluster first. On an older
  cluster, or when it cannot read the version, these screens are read every 2
  seconds, as before.
• :events names the volume, plugin or variable an event is about, and
  enter opens it.
• o on the job list opens the events of the job, its allocations,
  evaluations and deployments; on any list of allocations and on the tasks
  screen, the events of the allocation. The cluster filters them by key, so
  only those arrive, and a token that can read the job is enough. The screen
  stays on its job or allocation when you switch namespace. Not shown for a
  periodic or parameterized job.

Left for later: urga reads the version of the cluster from the agent's member
tags, which are empty when it talks to a client agent; there these screens
stay polled.

Closes #110
Closes #109

Checklist

• The test for this change was written first and failed
• make check is green
• The title is a conventional commit: feat(ui): ..., fix(nomad): ...