Move board relationships to dedicated storage

Summary:
Fixes T5476. Using edges to store which objects are on which board columns ends up being pretty awkward. In particular, it makes T4807 very difficult to implement.

Introduce a dedicated `BoardColumnPosition` storage.

This doesn't affect ordering rules (T4807) yet: boards are still arranged by priority. We just read which tasks are on which columns out of a new table.

Test Plan:
  - Migrated data, then viewed some boards. Saw exactly the same data.
  - Dragged tasks from column to column.
  - Created a task directly into a column.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5476

Differential Revision: https://secure.phabricator.com/D10160

src/applications/project/controller/PhabricatorProjectMoveController.php

@@ -57,27 +57,26 @@ final class PhabricatorProjectMoveController
       return new Aphront404Response();
     }
 
+    $positions = id(new PhabricatorProjectColumnPositionQuery())
+      ->setViewer($viewer)
+      ->withColumns($columns)
+      ->withObjectPHIDs(array($object_phid))
+      ->execute();
+
     $xactions = array();
 
-    $edge_type = PhabricatorEdgeConfig::TYPE_OBJECT_HAS_COLUMN;
-
-    $query = id(new PhabricatorEdgeQuery())
-      ->withSourcePHIDs(array($object->getPHID()))
-      ->withEdgeTypes(array($edge_type))
-      ->withDestinationPHIDs(array_keys($columns));
-
-    $query->execute();
-
-    $edge_phids = $query->getDestinationPHIDs();
-
     $xactions[] = id(new ManiphestTransaction())
       ->setTransactionType(ManiphestTransaction::TYPE_PROJECT_COLUMN)
-      ->setNewValue(array(
-        'columnPHIDs' => array($column->getPHID()),
-        'projectPHID' => $column->getProjectPHID()))
-      ->setOldValue(array(
-        'columnPHIDs' => $edge_phids,
-        'projectPHID' => $column->getProjectPHID()));
+      ->setNewValue(
+        array(
+          'columnPHIDs' => array($column->getPHID()),
+          'projectPHID' => $column->getProjectPHID(),
+        ))
+      ->setOldValue(
+        array(
+          'columnPHIDs' => mpull($positions, 'getColumnPHID'),
+          'projectPHID' => $column->getProjectPHID(),
+        ));
 
     $task_phids = array();
     if ($after_phid) {
@@ -86,6 +85,7 @@ final class PhabricatorProjectMoveController
     if ($before_phid) {
       $task_phids[] = $before_phid;
     }
+
     if ($task_phids) {
       $tasks = id(new ManiphestTaskQuery())
         ->setViewer($viewer)