Convert 'WidgetController' into a `<sidebar-content>` component which
uses `sidebar_content.html` as a template and convert the route for the
sidebar into a trivial template which just renders this component.

 * Move `widget-controller` to src/sidebar/components/sidebar-content.js

 * Move various helpers for the sidebar content template from the scope
   to the controller, in line with other components.

 * Pass the "auth" and "search" scope properties from the AppController
   instance explicitly to `<sidebar-content>`, since components do not
   inherit scope properties automatically.