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/maniphest/controller/ManiphestTaskEditController.php

@@ -387,10 +387,23 @@ final class ManiphestTaskEditController extends ManiphestController {
                 ->withPHIDs($task_phids)
                 ->execute();
 
-              $sort_map = mpull(
-                $column_tasks,
-                'getPrioritySortVector',
-                'getPHID');
+              if ($order == PhabricatorProjectColumn::ORDER_NATURAL) {
+                // TODO: This is a little bit awkward, because PHP and JS use
+                // slightly different sort order parameters to achieve the same
+                // effect. It would be unify this a bit at some point.
+                $sort_map = array();
+                foreach ($positions as $position) {
+                  $sort_map[$position->getObjectPHID()] = array(
+                    -$position->getSequence(),
+                    $position->getID(),
+                  );
+                }
+              } else {
+                $sort_map = mpull(
+                  $column_tasks,
+                  'getPrioritySortVector',
+                  'getPHID');
+              }
 
               $data = array(
                 'sortMap' => $sort_map,