Storypackage prototype switcher

If a story-package is available for a given article (i.e. it's been added to an event in the admin tool) it will be ajax'd into the article. Otherwise, normal related-trails behaviour happens.  

The admin switchboard turns on/off story-package view prototypes. If more than one is turned on, the first one wins. Any change takes a minute or so to take effect.

To override the switch mechanism, set a localStorage key of "gu.storypackage" in your browser to the name of a story package, e.g. "chronological02"

To add a new story-package prototype, add a route/controller/view etc, and another switch in the admin switchboard.