Make workboard CSS aware of the durable column

Summary: Fixes T8246. Workboards needed a tweak after the native-scrollbar compromise.

Test Plan: Viewed workboards with column open/closed, saw workspace size correctly.

Reviewers: chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8246

Differential Revision: https://secure.phabricator.com/D12906