Support natural ordering of workboards

Summary:
Ref T4807. This is probably a complete fix, but I'd be surprised if there isn't a little cleanup I missed.

When users drag tasks on a "natural"-ordered workboard, leave things where they put them.

This isn't //too// bad since a lot of the existing work is completely reusable (e.g., we don't need any new JS).

Test Plan:
  - Dragged a bunch of stuff around, it stayed where I put it after dropped and when reloaded.
  - Dragged stuff across priorities, no zany priority changes (in "natural" mode).
  - Created new tasks, they show up at the top.
  - Tagged new tasks, they show up at the top of backlog.
  - Swapped to "priority" mode and got sorting and the old priority-altering reordering.
  - Added tasks in priority mode.
  - Viewed task transactions for correctness/sanity.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: chad, epriestley

Maniphest Tasks: T4807

Differential Revision: https://secure.phabricator.com/D10182

src/applications/project/controller/PhabricatorProjectBoardViewController.php

@@ -178,6 +178,23 @@ final class PhabricatorProjectBoardViewController
       $task_map[$column_phid][] = $task_phid;
     }
 
+    // If we're showing the board in "natural" order, sort columns by their
+    // column positions.
+    if ($this->sortKey == PhabricatorProjectColumn::ORDER_NATURAL) {
+      foreach ($task_map as $column_phid => $task_phids) {
+        $order = array();
+        foreach ($task_phids as $task_phid) {
+          if (isset($positions[$task_phid])) {
+            $order[$task_phid] = $positions[$task_phid]->getOrderingKey();
+          } else {
+            $order[$task_phid] = 0;
+          }
+        }
+        asort($order);
+        $task_map[$column_phid] = array_keys($order);
+      }
+    }
+
     $task_can_edit_map = id(new PhabricatorPolicyFilter())
       ->setViewer($viewer)
       ->requireCapabilities(array(PhabricatorPolicyCapability::CAN_EDIT))