R1:a823d143b404

"Hide" columns instead of "Deleting" them

Summary:
Fixes T5342. Fixes T5161. Previously, we were a bit strict about deleting columns because you could orphan tasks. Let users recover these columns more easily so they can't shoot themselves in the foot.

- Change "Delete" language to "Hide".
- Add a button to let you see hidden columns.
- Remove restriction that you can only delete empty columns.

The new button is a little funky, but maybe it merges into the "Add Column" button and that becomes a…