Rewrite workboards to have way more bugs

Summary:
Ref T4900. Briefly:

• Much more layout and rendering is now done in Javascript.
• This should otherwise be identical to the behavior at HEAD, except that:
  • editing a task and removing the current board from it no longer removes the task; and
  • points still don't work.

However, this can now plausibly support realtime workboard updates and other complex state-based behaviors like points calculations in a future change.

Test Plan:

• Changed card covers.
• Moved cards.
• Sorted board by priority and natural.
• Added new cards.
• Edited cards in place.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4900

Differential Revision: https://secure.phabricator.com/D15234