"Hide" columns instead of "Deleting" them
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 dropdown with board actions? The rest of this feels OK to me.
Test Plan: See screenshot.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D9719