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.
Reviewed By: epriestley
Maniphest Tasks: T10364
Differential Revision: https://secure.phabricator.com/D16533