Allow the Default/Backlog column to be renamed

Summary: Fixes T5101. There's no technical reason not to allow this, it just took a little extra work so I didn't do it originally.

Test Plan: Renamed "Backlog", un-renamed it. Tried to hide it.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5101

Differential Revision: https://secure.phabricator.com/D9721