Extract core rendering functionality from the mw.popups.renderer and
mw.popups.renderer.article objects.
For now, render and show the preview when the user dwells on and
abandons a link respectively.
Supporting changes:
* Add mw.popups.wait, which is sugar around window.setTimeout.
* action.response -> action.result in the FETCH_END case of the preview
reducer.
Change-Id: I14b437e7c2f55b988837fcb2800dd61a23c29a01
