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