R1:9a8019d4a98b

Modularize workboard column orders

Summary:
Depends on D20267. Depends on D20268. Ref T10333. Currently, we support "Natural" and "Priority" orders, but a lot of the particulars are pretty hard-coded, including some logic in `ManiphestTask`.

Although it's not clear that we'll ever put other types of objects on workboards, it seems generally bad that you need to modify `ManiphestTask` to get a new ordering.

Pull the ordering logic out into a `ProjectColumnOrder` hierarchy instead, and let each ordering…