tree: don't expand all cycles if the resulting node count is too high

[oliver-sanders]

The tree view will auto expand all cycle points when it is loaded. This is usually helpful.

If you have a workflow with a large runahead limit, however, the number of family/task/job rows could get rather large. Rather than auto-expanding all cycle points, we could start with the newest/oldest cycle point and expand the cycles up to a configured node limit and leave the remainder collapsed.

Note, this may interact with search/filter functionality, see https://github.com/cylc/cylc-ui/issues/1228