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 parent project tag.
• (Plenty of UI / design stuff to adjust.)

Test Plan:

• Dragged stuff between milestone columns.
• Used a normal workboard.
• Wasn't able to find any egregiously bad cases that did anything terrible.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10010

Differential Revision: https://secure.phabricator.com/D15171