R1:90a0459821ea

Roughly implement milestone columns on workboards

Summary:
Ref T10010. These aren't perfect but I think (?) they aren't horribly broken.

- When a project is a parent project, destroy (as far as the user can tell) any custom columns.
- When a project has milestones, automatically generate columns on the project's workboard (if it has a workboard).
- When you move tasks between milestones, add the proper milestone tag.
- When you move tasks out of milestones back into the backlog, add the proper…