board view of project for moving items between milestones. PMT #92269

Very sparse wire-frame kind of interface for now. Whole thing is behind
the 'project_board' feature flag.
only show open milestones on board page

Thought about it and, yeah, by default it really is better to only have open milestones
Merge pull request #169 from ccnmtl/board-view

board view of project for moving items between milestones. PMT #92269