Roughly implement milestone columns on workboards
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 parent project tag.
- (Plenty of UI / design stuff to adjust.)
- Dragged stuff between milestone columns.
- Used a normal workboard.
- Wasn't able to find any egregiously bad cases that did anything terrible.
Reviewed By: chad
Maniphest Tasks: T10010
Differential Revision: https://secure.phabricator.com/D15171