Directly attach column/header classes in the column definition

[bhollis]

The Organizer is set up a bit weirdly today - classes are implicitly attached to columns by name, which makes it difficult to keep track of what's used where. We also attach the entire cell class to the column headers, which is rarely appropriate. This also makes it hard to reuse one class across multiple columns.

This PR changes to having an explicit className and headerClassName property on each column that we can assign from a CSS module import, just like all our other classes.