"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 dropdown with board actions? The rest of this feels OK to me.

Test Plan: See screenshot.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5342, T5161

Differential Revision: https://secure.phabricator.com/D9719