R1:46ed8d4a5ea7

On Workboards, sort groups by "natural order", not subpriority

Summary:
Depends on D20263. Ref T10333. I want to add groups like "Assignee" to workboards. This means you may have several tasks grouped under, say, "Alice".

When you drag the bottom-most task under "Alice" to the top, what does that mean?

Today, the only grouping is "Priority", and it means "change the task's secret/hidden global subpriority". However, this seems to generally be a somewhat-bad answer, and is quite complex. It also doesn't…