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.