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 closed duplicate tasks.
  • Con: Dragging tasks to "Duplicate" works, but is silly.
• Since boards show "open tasks" by default, dragging stuff to a closed status and then reloading the board causes it to vanish. This is kind of how everything works, but more obvious/defaulted on "Status".

These issues might overwhelm its usefulness, but there isn't much cost to nuking it in the future if feedback is mostly negative/confused.

Test Plan: Grouped a workboard by status, dragged stuff around.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10333

Differential Revision: https://secure.phabricator.com/D20277