R1:a400d82932a6

Add "Group by Status" to Workboards

Summary:
Depends on D20276. Ref T10333. This one is a little bit rough/experimental, and I'm sort of curious what feedback we get about it. Weird stuff:

- All statuses are always shown, even if the filter prevents tasks in that status from appearing (which is the default, since views are "Open Tasks" by default).
- Pro: you can close tasks by dragging them to a closed status.
- Con: lots of empty groups.
- The "Duplicate" status is shown.
- Pro: Shows…