Begin generalizing Javascript for Workboard state handling

Summary:
Ref T4900. Broadly, workboard state management is fairly ad-hoc now, which makes things like this (where some kind of edit affects global state) difficult:

  - Updating points header to reflect a sum change after dragging a task.
  - Changing progress bars after editing a task to change resolution or points value.
  - Moving a card to the correct column after editing it and changing subprojects/iterations.
  - Responding to real-time notifications about other users moving cards.

This begins rewriting the code in a way that can better accommodate these kinds of far-reaching state update.

This change just moves cover image stuff. I'll continue moving features one at a time until boards work better.

Test Plan: Updated some cover images.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4900

Differential Revision: https://secure.phabricator.com/D15224

src/applications/project/controller/PhabricatorProjectBoardViewController.php

@@ -231,7 +231,12 @@ final class PhabricatorProjectBoardViewController
 
     $board = id(new PHUIWorkboardView())
       ->setUser($viewer)
-      ->setID($board_id);
+      ->setID($board_id)
+      ->addSigil('jx-workboard')
+      ->setMetadata(
+        array(
+          'boardPHID' => $project->getPHID(),
+        ));
 
     $behavior_config = array(
       'boardID' => $board_id,