Preview the effects of a drag-and-drop operation on workboards

Summary:
Ref T10335. Ref T5474. When you drag-and-drop a card on a workboard, show a UI hint which lists all the things that the operation will do.

This shows: column moves; changes because of dragging a card to a different header; and changes which will be caused by triggers.

Not implemented here:

  - Actions are currently shown even if they have no effect. For example, if you drag a "Normal" task to a different column, it says "Change priority to Normal.". I plan to hide actions which have no effect, but figuring this out is a little bit tricky.
  - I'd like to make "trigger effects" vs "non-trigger effects" a little more clear in the future, probably.

Test Plan:
Dragged stuff between columns and headers, and into columns with triggers. Got appropriate preview text hints previewing what the action would do in the UI.

(This is tricky to take a screenshot of since it only shows up while the mouse cursor is down.)

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T10335, T5474

Differential Revision: https://secure.phabricator.com/D20299

src/applications/project/controller/PhabricatorProjectBoardViewController.php

@@ -540,8 +540,8 @@ final class PhabricatorProjectBoardViewController
       ->setExcludedProjectPHIDs($select_phids);
 
     $templates = array();
-    $column_maps = array();
     $all_tasks = array();
+    $column_templates = array();
     foreach ($visible_columns as $column_phid => $column) {
       $column_tasks = $column_phids[$column_phid];
 
@@ -606,18 +606,28 @@ final class PhabricatorProjectBoardViewController
             'pointLimit' => $column->getPointLimit(),
           ));
 
+      $card_phids = array();
       foreach ($column_tasks as $task) {
         $object_phid = $task->getPHID();
 
         $card = $rendering_engine->renderCard($object_phid);
         $templates[$object_phid] = hsprintf('%s', $card->getItem());
-        $column_maps[$column_phid][] = $object_phid;
+        $card_phids[] = $object_phid;
 
         $all_tasks[$object_phid] = $task;
       }
 
       $panel->setCards($cards);
       $board->addPanel($panel);
+
+      $drop_effects = $column->getDropEffects();
+      $drop_effects = mpull($drop_effects, 'toDictionary');
+
+      $column_templates[] = array(
+        'columnPHID' => $column_phid,
+        'effects' => $drop_effects,
+        'cardPHIDs' => $card_phids,
+      );
     }
 
     $order_key = $this->sortKey;
@@ -661,9 +671,9 @@ final class PhabricatorProjectBoardViewController
       'headers' => $headers,
       'headerKeys' => $header_keys,
       'templateMap' => $templates,
-      'columnMaps' => $column_maps,
       'orderMaps' => $vector_map,
       'propertyMaps' => $properties,
+      'columnTemplates' => $column_templates,
 
       'boardID' => $board_id,
       'projectPHID' => $project->getPHID(),