Etherpad of APZ Etherpads

APZ planning: https://firefoxgraphics.github.io/DesignDocs/etherpad-migration/apz-planning

Async keyboard scrolling
 Original discussion: https://firefoxgraphics.github.io/DesignDocs/etherpad-migration/async-keyboard-scrolling
 More recent discussion: https://firefoxgraphics.github.io/DesignDocs/etherpad-migration/apz-keyboard