Add a toggle, sticky pref, for Conpherence Widget Pane

Summary: This adds a "column" icon into crumbs, like in workboards, for expanding or hiding the "Widget Pane". This is per user sticky and defaults to off.

Test Plan: View a Conpherence Room, see no widgets by default. Toggle it on, see widget. Reload page, see widget stick. Verify mobile, tablets ignore hiding.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T10364

Differential Revision: https://secure.phabricator.com/D16533