Make default and hidden columns in Workboards more clear

Summary: Fixes T6469. Changes the default icon into text instead. Added the text to hidden boards and now display when reordering as well.

Test Plan:
Moved a bunch of columns, tested reordering. Seems more clear.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6469

Differential Revision: https://secure.phabricator.com/D10784