web: extract redesign-toggle-decorator from redesign-toggle-toolbar

Created by: valerybugakov

To apply redesign-theme in Chromatic screenshots correctly, related logic is extracted from the toolbar component into a decorator function. Also, allow useObservable to use BehaviorSubject.value as initial value.

Tested here: https://github.com/sourcegraph/sourcegraph/pull/20516. Closes #20513.