Move UpdateSubscriber to Root component from NavbarUser
Currently the UpdateSubscriber component is rendered from within NavbarUser: https://gitlab.coko.foundation/pubsweet/pubsweet/blob/master/packages/components/Navigation/NavbarUser.jsx#L9
As per RFC #318 (closed), we should move it out of there into the Root component.
The SSE should be controlled by a corresponding configuration property (per instance configurability)