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.
• Enabled a board.

Reviewers: chad

Reviewed By: chad

Subscribers: mbishopim3

Maniphest Tasks: T7410

Differential Revision: https://secure.phabricator.com/D15215