R1:0bf3519045b2

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…