Big change to how the AnchoredBox component is structured so that it can be used in more than just one spot. Now composed of the wrapper Anchored, with two children AnchoredTrigger and AnchoredBox, each of which can have their own arbitrary children.
Next steps will involve renaming the component file to Anchored.jsx, moving most styling out of the attached stylesheet (and into brewRenderer.less), and adding props to pass in Anchor Positioning properties.
Adds a new AnchoredBox component that is functionally a clone of the "saving error" notifications, but drops a lot of the JS in favor of the new (chrome-only!) CSS Anchor Positioning API. In subsequent commits, either alternate styling or a polyfill will be added non-supported browsers.
The box contains a few inputs that modify the CSS applied to `.pages`, most critically a "start on right" toggle for the Facing Pages mode.
