Refactoring the AnchoredBox component because I wanted to set focus on the trigger button when the expanded box was closed. Wrapping the trigger into it's own component, with forwardRef, allows for passing the `ref` to the actual DOM node. Then the `.focus()` method will work on it.
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.
