Plugin System

As a developer, I want the ability to create plugins for the DA editor so that I can add functionality not covered by out of the box DA features.

## Criteria of acceptance
* Move AEM Assets to library panel
* Plugins are first-class citizens inside library
* Ability to bi-directionally send HTML between plugin & editor
* HTML from plugin is sanitized from styles and scripts

## High level concepts (via @nc-andreashaller)
- move .da/config to sidekick/config.json
- make custom plugins and built-in plugins look the same, elevate them in the menu to make them first class citizens
- da-library/helpers.json -> getLibraryList
- aemsites/da-block-collection
- da-preview.js
- parameters: html, path, (selection snippet)
- out: "replace event" html to update editor, think about "insert" for snippet