R1:3682cc9bb2e8

Allow workboards to be disabled, hiding "(Backlog)" column annotations

Summary:
Fixes T7410.

- Adds a "Disable Workboard" action to the "Manage Backlog" menu.
- We'll probably move this somewhere else if/when that column gets too messy.
- Disabling a board hides it, prevents it from being recreated by non-editors, and hides the "Project (Backlog)" annotations.
- Resotring a board puts it back in pristine condition.

Test Plan:
- Disabled a board.
- Verified "(Backlog)" annotations vanished.